A fashionable set in function that's senseless (7)
I believe the answer is:
asinine
'senseless' is the definition.
(thesaurus)
'a fashionable set in function' is the wordplay.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'set in' is an insertion indicator.
'function' becomes 'sine' (mathematical function).
'in' placed within 'sine' is 'sinine'.
'a'+'sinine'='ASININE'
'that's' acts as a link.
(Other definitions for asinine that I've seen before include "Donkey-like" , "Senseless" , "Idiotic" , "Is Annie? (Anag)" , "Stupid (as a donkey?)" .)