Swelling damaged plum (4)
I believe the answer is:
lump
'swelling' is the definition.
('lump' can be a synonym of 'swelling')
'damaged plum' is the wordplay.
'damaged' indicates an anagram.
'plum' is an anagram of 'LUMP'.
(Other definitions for lump that I've seen before include "Small solid mass" , "Hunk - chunk" , "Protuberance or swelling" , "Small shapeless mass" , "Irregularly shaped piece" .)