Bad habit's very cool (4)
I believe the answer is:
vice
'bad habit's' is the definition.
(thesaurus)
'very cool' is the wordplay.
'very' becomes 'v' (abbreviation).
'cool' becomes 'ice' (icing is a kind of cooling).
'v'+'ice'='VICE'
(Other definitions for vice that I've seen before include "Gripper - sin" , "may have a powerful grip" , "Prefix for deputy to, say, chairman" , "Sin; weakness" , "item in workshop" .)