Bitter free on account (5)
I believe the answer is:
acrid
'bitter' is the definition.
(thesaurus)
'free on account' is the wordplay.
'free' becomes 'rid' ('rid' can be a synonym of 'free').
'on' means one lot of letters go next to another.
'account' becomes 'ac'.
'rid' after 'ac' is 'ACRID'.
(Other definitions for acrid that I've seen before include "Sarcastic" , "Stinging" , "Having a bitter taste or small, like smoke" , "causing corrosion" , "Sharp - biting" .)