He may study an insect and its changing form (9)
I believe the answer is:
scientist
'he may study an' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'insect and its changing form' is the wordplay.
I cannot quite see how this works, but
an anagram of 'insect' is 'scient' which is located in the answer.
an anagram of 'its' is 'ist' which is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for scientist that I've seen before include "someone in laboratory?" , "Person conducting experiments" , "It's insect, strangely, this learned person knows about" , "Eg, a chemist" , "one in the laboratory" .)