Tart's free on account (5)
I believe the answer is:
acrid
'tart's' is the definition.
(I've seen this before)
'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' put after 'ac' is 'ACRID'.
(Other definitions for acrid that I've seen before include "Stinging" , "Having a bitter smell or taste" , "Bitterly pungent" , "Unpleasantly strong-smelling" , "Strong and sharp" .)