Chop topped with cold champ (4)
I believe the answer is:
chew
'champ' is the definition.
(I know that champ is a more specific form of the action chew)
'chop topped with cold' is the wordplay.
'chop' becomes 'hew' (to hew is to chop or cut down).
'topped with' says to put letters next to each other.
'cold' becomes 'c' (eg on taps).
'hew' after 'c' is 'CHEW'.
(Other definitions for chew that I've seen before include "work with canines" , "Masticate, grind between teeth" , "Ruminate" , "champ" , "Gnaw" .)