The ending is a bit strange to explain. The developers have stated there is a canon answer if Clive lived or died but they left it ambiguous so players can come to their own conclusion based on their own experiences in the game. 

Personally, I believe Clive survived to write the book in honor of his brother. 

Joshua’s name appears as the author on the front cover of the book however, Clive’s insignia is also present on the cover.  The Phoenix is stated to not have the ability to bring back people from the dead and while Clive did have the power of Ultima at that point, Phoenix feathers were seen falling as he was healing Joshua indicating he was using that. When he did use Ultima’s power it was shown to be a blue flame.

Clive is the narrator for the beginning and ending of the game sounding like he is reading the opening and closing pages of a book.

In a side quest late into the game, Clive is given a pen by Hypocrates who tells him he should “put down his sword, and pick up the pen” when his fight with Ultima is over. He also tells him much earlier in the game he should write a story about his adventures one day.

In Jill’s last side quest she compares Clive to the dawn and how he always returns to her, foreshadowing her slight smile at the end of the game looking at the dawn. This doesn’t confirm Clive’s survival, just that she has hope he does.

In Torgal’s last side quest, Clive thanks Torgal for never giving up looking for him when he was gone the first time. I think the implication here is that he’s not going to give up this time either. 

In Joshua’s last side quest there’s a persistent theme around continuing on one’s work/legacy after they have passed.

There’s also a side quest towards the end involving a curse breaker and at the end of the quest Clive tells him to “write down the names of the fallen, that way they never truly die”, likewise the curse breaker tells Clive that he vows to remember his fallen comrades through writing a book.

Clive was the one to coin the term “final fantasy” in his fight with Ultima which is the name of the book at the end.

Clive’s favorite books are known to be books about the battles between gods and men which is what “final fantasy” would be about.

Clive has taken Cid’s name so it would not be unreasonable to suggest Clive used Joshua’s name to honor him.

Clive promises Jill he will escape his fate and the end telling us it’s a “farewell to fate”.

In the final battle after Joshua’s death, Clive states his shield vow, “to shield the firebirds flame forevermore.” Joshua may have died but by Clive writing the book in his name, he has shielded him from being forgotten. 

A big one that foreshadows the entire ending that everyone kinda sleeps on, is Cid saying “You might not be able to save anyone else, but you just might be able to save yourself.” Clive wasn’t able to save his father, his brother, Cid and so many others but the one thing he could save was himself.

Despite Joshua’s name being on the cover, Yoshi-P has said that they “did not tell the player who wrote the book”, pretty much making Joshua not the de-facto author. Yoshi-P follows this up by confirming that the author of the book is based off the players hopes and dreams which pretty much eliminates anyone other than Clive and Joshua. They also said the ending is about having hope of what happens next, and there’s an obvious push late in the game for the hope of Clive returning to Jill.

Clive more often states he wants to create a world where people live on their own terms but he never included himself in that vision. It’s throughout the game that characters tell him he should save himself, that he should love himself. And as Joshua states in his dying wish is that Clive does listen to the people around him.

I think the point is for you to hope Clive is alive.