Bill gets free bitter (5)
I believe the answer is:
acrid
'bitter' is the definition.
(I've seen this before)
'bill gets free' is the wordplay.
'bill' becomes 'ac' (short for account).
'gets' is a charade indicator (letters next to each other).
'free' becomes 'rid' (to be free of something is to be rid of it).
'ac'+'rid'='ACRID'
(Other definitions for acrid that I've seen before include "Strong and sharp-smelling" , "Bitter - pungent" , "irritating" , "Unpleasantly bitter" , "Pungent; caustic" .)