Dumpling gained weight (3,3)
I believe the answer is:
won ton
'dumpling' is the definition.
(I've seen this before)
'gained weight' is the wordplay.
'gained' becomes 'won' (I've seen this before).
'weight' becomes 'ton' (ton is a kind of weight).
'won'+'ton'='WON TON'
(Other definitions for won ton that I've seen before include "'Chinese dumpling served in soup (3,3)'" , "snack" .)