Knock back roll filled with cold ham? (5)
I believe the answer is:
actor
'ham?' is the definition.
(ham can mean a bad actor)
'knock back roll filled with cold' is the wordplay.
'knock back' says the letters should be written in reverse.
'roll' becomes 'rota' (rota is a kind of roll).
'filled with' indicates putting letters inside.
'cold' becomes 'C' (eg on taps).
'rota' placed around 'c' is 'rotca'.
'rotca' backwards is 'ACTOR'.
(Other definitions for actor that I've seen before include "I pretend" , "Croat (anag.)" , "Theatrical performer" , "eg Anthony Hopkins" , "One taking part" .)