Case of trendy attitude (8)
I believe the answer is:
'case' is the definition.
(I know that case can be written as instance)
'trendy attitude' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'attitude' becomes 'stance' (stance is a kind of attitude).
'of' acts as a link.
(Other definitions for instance that I've seen before include "Case or example from ancients" , "A case or example of something" , "Example or single occurrence of something" , "Particular case; cite" , "'Example, illustration (8)'" .)