Treatment for one largely dull, small swelling (6)
I believe the answer is:
nodule
'small swelling' is the definition.
(I've seen this before)
'treatment for one largely dull' is the wordplay.
'treatment for' indicates an anagram (I've seen this in other clues).
'largely' means to remove the last letter.
'dull' with its last letter taken off is 'dul'.
'one'+'dul'='onedul'
'onedul' is an anagram of 'NODULE'.
(Other definitions for nodule that I've seen before include "Small round lump" , "A small rounded lump" , "Little round lump" , "Small lump or swelling" , "Small knot" .)