Portion of plum pudding (4)
I believe the answer is:
lump
'portion' is the definition.
(thesaurus)
'of plum pudding' is the wordplay.
'of' says the answer is hidden in the clue.
'LUMP' is hidden within 'plum pudding'.
(Other definitions for lump that I've seen before include "Small shapeless mass" , "Shapeless mass - swelling" , "Piece of sugar, coal etc" , "Chunk, clod, nodule" , "Irregular mass" .)