Helping traitor? I have no comeback (6)
I believe the answer is:
ration
'helping' is the definition.
(a ration is a serving or helping)
'traitor? i have no comeback' is the wordplay.
'traitor?' becomes 'rat' (rat can mean a treacherous person).
'have' means one lot of letters go next to another.
'comeback' shows that the letters should be reversed in order.
'no' reversed gives 'on'.
'rat'+'i'+'on'='RATION'
(Other definitions for ration that I've seen before include "Portion, share" , "Food allowance" , "Limit supply in time of scarcity" , "Parcel out scarce food, say" , "A measured portion in times of scarcity" .)