Student less fat around end of September (7)
I believe the answer is:
learner
'student' is the definition.
(someone who is learning)
'less fat around end of september' is the wordplay.
'less fat' becomes 'leaner' ('lean' can be the opposite of 'fat' so perhaps 'leaner' is reasonable for 'less fat').
'around' means one lot of letters goes inside another.
'end of' suggests the final letters.
The last letter of 'september' is 'r'.
'leaner' placed around 'r' is 'LEARNER'.
(Other definitions for learner that I've seen before include "Acquirer of knowledge" , "Student (of driving?)" , "neophyte" , "One with an L-plate" , "who needs plates" .)