Plums’ unusual swellings (5)
I believe the answer is:
lumps
'swellings' is the definition.
('lump' can be a synonym of 'swelling')
'plums' unusual' is the wordplay.
'unusual' is an anagram indicator.
'plums' with letters rearranged gives 'LUMPS'.
(Other definitions for lumps that I've seen before include "Clods or swellings" , "Puts up with" , "Compact masses" , "Chunks" .)