Finish filling crack with it (6)
I believe the answer is:
trendy
'with it' is the definition.
(thesaurus)
'finish filling crack' is the wordplay.
'finish' becomes 'end' ('end' can be a synonym of 'finish').
'filling' is an insertion indicator.
'crack' becomes 'try' (I've seen this in other clues).
'end' put inside 'try' is 'TRENDY'.
(Other definitions for trendy that I've seen before include "all the rage" , "Voguish" , "Fashionable, in style" , "With-it" , "Chic (colloq.)" .)