A dollop of plum pudding! (4)
I believe the answer is:
lump
'a dollop' is the definition.
(I have seen 'A dollop' mean 'lump' so perhaps 'dollop' could also mean 'lump')
'of plum pudding' is the wordplay.
'of' indicates the answer is hidden within the clue.
'LUMP' is hidden in the letters of 'plum pudding'.
(Other definitions for lump that I've seen before include "Gross" , "Swallow" , "Node" , "growth" , "Small heap, swelling" .)