Fine summer? Put in one more particular (7)
I believe the answer is:
faddier
'more particular' is the definition.
(faddy can mean particular or fussy)
'fine summer? put in one' is the wordplay.
'fine' becomes 'f' (abbreviation e.g. condition of old coins, books).
'summer?' becomes 'adder' (someone who adds or does sums).
'put in' indicates putting letters inside (instruction to solver).
'one' becomes 'i' (Roman numeral).
'adder' going around 'i' is 'addier'.
'f'+'addier'='FADDIER'
(Another definition for faddier that I've seen is " more fussy about eating".)