Plum's unusual swelling (4)
I believe the answer is:
lump
'swelling' is the definition.
(I know that swelling can be written as lump)
'plum's unusual' is the wordplay.
'unusual' indicates anagramming the letters.
'plum' is an anagram of 'LUMP'.
(Other definitions for lump that I've seen before include "Chunk, clod, nodule" , "Small shapeless mass" , "Piece of sugar, coal etc" , "Small solid mass" , "Large piece without defined shape" .)