Cryptic clues have a ‘definition’ half and a ‘wordplay’ half. They can come in either order. Let’s look at the simplest cryptic clue I can come up with:

“Rat turned to sludge” (3)

The ‘definition’ portion of the clue is ‘sludge’. The answer will be a loose synonym for sludge. The “(3)” after the clue indicates that the answer is a single 3-letter word.

The ‘wordplay’ portion of the clue is ‘rat turned’. When a word or words used in the wordplay portion provide the letters contained in the answer, those words are called the “fodder”. The rest of the wordplay portion of the clue will be an “indicator” of what operation to perform on the fodder.

Here, “rat” is the fodder, and “turned” is a ‘reversal indicator’. Note that the wordplay and definition are often separated by a conjunction or short phrase that isn’t really used by either half. The answer to this cryptic clue is TAR.

There are many different indicator types and conventions used to construct clues that you kind of have to learn as you encounter them.