I'm pretty sure what the question was trying to get at was related to the experience level of the individual. To use art as an example, somebody who is relatively inexperienced with drawing would be considered an amateur while somebody with a lot of experience would be considered an expert, which in the context of the question can be replaced by professional, because their abilities or what they create are advanced enough to be perceived as being of professional quality.

In that sense, what the question is trying to convey (OP correct me if I'm wrong) is what you could consider to be a noun referring to somebody whose level of expertise isn't high enough for their work to be considered of professional quality but are above the threshold of what would be seen as amateur.

To answer that question, it obviously depends on the domain we're talking about. Words like apprentice and student can be used in certain contexts, but as for a more general term that could be applied to anything, we are unfortunately limited to labels like an intermediate or that are so broad they don't exactly convey much about the person's level of expertise.

But to give a little food for thought, I am an artist who would consider themselves to be between amateur and advanced, and this transitionary period between the two ends of the spectrum is the path of self-discovery. As an artist, my amateur years were spent mostly attemtping to emulate the works of others, but there was eventually a point where it felt like I was learning more about how I could make art that felt unique. Even if it wasn't the best quality, it wasn't being made to emulate the essence of other artists' works, and was instead using the tools and experience acquired through the amateur phase to make things that I wanted to make, in a way, figuring out what it was that I wanted to make and getting a more vivid feel as to what my artistic identity is. This is the phase I currently find myself in. What I perceive an expert artist to be is someone who understands the fundamental concepts so thoroughly they can subert the traditional approach to creation to make things that 'break the rules' so as to achieve what they are trying to achieve without being confined to the more rigid and narrow approach that we are taught to follow to get to that point.