Bill has very cool opinion (6)
I believe the answer is:
advice
'opinion' is the definition.
(I've seen this before)
'bill has very cool' is the wordplay.
'bill' becomes 'ad' (advertisement stuck on wall).
'has' is a charade indicator (letters next to each other).
'very' becomes 'v' (abbreviation).
'cool' becomes 'ice' (icing is a kind of cooling).
'ad'+'v'+'ice'='ADVICE'
(Other definitions for advice that I've seen before include "Suggestion for appropriate course of action" , "Counselling" , "Supposed words of wisdom" , "Proposal for an appropriate course of action" , "tip" .)