Potato with small pudding (4)
I believe the answer is:
spud
'potato' is the definition.
(I've seen this before)
'with small pudding' is the wordplay.
'with' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pudding' becomes 'pud' ('pud' can be a synonym of 'pudding').
's' put next to 'pud' is 'SPUD'.
(Other definitions for spud that I've seen before include "Starchy bit of food" , "King Edward, e.g." , "One might have an eye" , "King Edward, say (slang)" , "veg" .)