Composer’s into diet control, has a bit of zucchini (9)
I believe the answer is:
donizetti
'composer's' is the definition.
(I've seen this before)
'into diet control has a bit of zucchini' is the wordplay.
'control' indicates anagramming the letters.
'has' is an insertion indicator.
'a bit of' suggests taking the first letters.
The initial letter of 'zucchini' is 'z'.
'into'+'diet'='intodiet'
'intodiet' with letters rearranged gives 'donietti'.
'donietti' enclosing 'z' is 'DONIZETTI'.
(Other definitions for donizetti that I've seen before include "Italian composer" , "writer of noted works" .)