Attempt to keep tail cool (6)
I believe the answer is:
trendy
'cool' is the definition.
(both can mean fashionable)
'attempt to keep tail' is the wordplay.
'attempt' becomes 'try' (synonyms).
'to keep' is an insertion indicator.
'tail' becomes 'end' (the tail of something can mean the end).
'try' enclosing 'end' is 'TRENDY'.
(Other definitions for trendy that I've seen before include "Fashionable, in style" , "following fashion" , "'Hip, fashionable (6)'" , "Hot" , "following latest convention" .)