Tip trailer: a bad habit (6)
I believe the answer is:
advice
'tip' is the definition.
(I've seen this before)
'trailer a bad habit' is the wordplay.
'trailer' becomes 'ad' (I've seen this in other clues).
'a bad habit' becomes 'vice' (I've seen this before).
'ad'+'vice'='ADVICE'
(Other definitions for advice that I've seen before include "Supposedly wise words" , "Opinion given about what to do" , "Warning - guidance" , "tip" , "'Counsel, guidance (6)'" .)