Fashionable, only diamonds wrong
I believe the answer is:
injustice
'wrong' is the definition.
(I've seen this before)
'fashionable only diamonds' is the wordplay.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'only' becomes 'just' ('just' can be a synonym of 'only').
'diamonds' becomes 'ice' (I've seen this before).
'in'+'just'+'ice'='INJUSTICE'
(Other definitions for injustice that I've seen before include "Unfairness" , "Lack of fairness" , "It's unfair" , "wrong" .)