Devour ham, say, heading off (3)
I believe the answer is:
eat
'devour' is the definition.
(thesaurus)
'ham say heading off' is the wordplay.
'ham say' becomes 'meat' (ham is a kind of meat).
'heading off' means to remove the first letter.
'meat' with its initial letter taken away is 'EAT'.
(Other definitions for eat that I've seen before include "Munch" , "and 27: Be humiliated" , "Devour" , "Do cats - bats? Do bats - cats? (Alice in Wonderland)" , "Consume for tea" .)