Unusual plum is swelling (4)
I believe the answer is:
lump
'is swelling' is the definition.
(I've seen this before)
'unusual plum' is the wordplay.
'unusual' is an anagram indicator.
'plum' anagrammed gives 'LUMP'.
(Other definitions for lump that I've seen before include "Swallow" , "Small shapeless mass" , "Large piece without defined shape" , "Put up regardless" , "growth" .)