Ham on free rein (8)
I believe the answer is:
overplay
'ham' is the definition.
('overplay' can be a synonym of 'ham')
'on free rein' is the wordplay.
'on' becomes 'over' (I've seen this before).
'free rein' becomes 'play' ('play' can be a synonym of 'free rein').
'over'+'play'='OVERPLAY'
(Other definitions for overplay that I've seen before include "Give excess emphasis to" , "Exaggerate the importance of" , "Embroider" , "Complicate or exaggerate (something)" .)