It's very small at first, doctor (9)
I believe the answer is:
anatomist
'doctor' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'it's very small at first' is the wordplay.
I cannot really understand how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is present in the answer.
'it's' could be 'it' (I've seen this in other clues) and 'it' is found within the remaining letters.
'at' is found in the answer.
'first' could be 'a' and 'a' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for anatomist that I've seen before include "One studying bodily structure" , "one studies bodies" , "Body expert" , "One studying bodies" , "Biological expert" .)