Have piece of cake, a treat (3)
I believe the answer is:
eat
'have' is the definition.
(I know that eat is a more specific form of the action have)
'piece of cake a treat' is the wordplay.
'piece of' indicates the answer is hidden within the clue.
'EAT' can be found hidden inside 'cake a treat'.
(Other definitions for eat that I've seen before include "Use a knife and fork" , "Make food disappear" , "Bite, chew and swallow" , "Munch" , "Be corrosive" .)