Cook undersized fish (3)
I believe the answer is:
fry
This could be a double definition.
'cook' is the first definition.
(frying is a type of cooking)
'undersized fish' is the second definition.
'fry' can be an answer for 'fish' (I've seen this before). I am not sure about the 'undersized' bit.
Can you help me to learn more?
(Other definitions for fry that I've seen before include "Cook in oil" , "Elizabeth > Cook" , "Newly-hatched fishes for the pan?" , "Christopher" , "Small fish for the pan" .)