Head of couture to demolish fashion (5)
I believe the answer is:
craze
'fashion' is the definition.
(craze is a kind of fashion)
'head of couture to demolish' is the wordplay.
'head of' suggests taking the first letters.
'to demolish' becomes 'raze' (I've seen this in another clue).
The initial letter of 'couture' is 'c'.
'c'+'raze'='CRAZE'
(Other definitions for craze that I've seen before include "Widespread fad" , "Fine crack" , "Wild enthusiasm" , "Mania" , "Fad, short-lived enthusiasm" .)