Attempt to hold butt in (6)
I believe the answer is:
trendy
'in' is the definition.
('in' can mean trendy or fashionable)
'attempt to hold butt' is the wordplay.
'attempt to' becomes 'try' (I've seen this in other clues).
'hold' indicates putting letters inside.
'butt' becomes 'end' (eg the butt or end of a cigarette).
'try' enclosing 'end' is 'TRENDY'.
(Other definitions for trendy that I've seen before include "In accordance with the latest fad" , "Informally, following the fashions" , "With it, a la mode" , "'Hip, fashionable (6)'" , "Modish" .)