"Sweets put weight on" is the comeback! (8)
I believe the answer is:
desserts
'sweets' is the definition.
('dessert' can be a synonym of 'sweet')
'put weight on is the comeback' is the wordplay.
'put weight on' becomes 'stress' (I've seen this before).
'is the' becomes 'ed' (I am not sure about this - if you are sure you should believe this answer much more).
'comeback' says the letters should be written backwards.
'stress'+'ed'='stressed'
'stressed' backwards is 'DESSERTS'.
Can you help me to learn more?
(Other definitions for desserts that I've seen before include "That may be cobblers" , "fools, perhaps?" , "Sweet courses of meal" , "Final courses" , "Sweets" .)