Ingredient of pudding, a raw fish (3)
I believe the answer is:
gar
'fish' is the definition.
(gar is a kind of fish)
'ingredient of pudding a raw' is the wordplay.
'ingredient of' says the answer is hidden in the clue.
'GAR' is hidden in the letters of 'pudding a raw'.
(Other definitions for gar that I've seen before include "Creature" , "A fish; cloth scrap (reversed)" , "something fishy here" , "Type of fish" , "swimmer" .)