Plain about account for ham (7)
I believe the answer is:
overact
'ham' is the definition.
('overact' can be a synonym of 'ham')
'plain about account' is the wordplay.
'plain' becomes 'overt' (I've seen this before).
'about' means one lot of letters goes inside another.
'account' becomes 'ac'.
'overt' placed around 'ac' is 'OVERACT'.
'for' acts as a link.
(Other definitions for overact that I've seen before include "behave exaggeratedly" , "Be excessive like fulsome thespian" , "Play part exaggeratedly" , "Perform hammily" , "Cartoon twenty four across" .)