Inside of crustacean’s not cooked (3)
I believe the answer is:
raw
'not cooked' is the definition.
('raw' is the opposite to 'cooked')
'inside of crustacean's' is the wordplay.
'inside of' indicates the central letters.
'crustacean' becomes 'prawn' (prawn is a kind of crustacean).
The middle of 'prawn' is 'RAW'.
(Other definitions for raw that I've seen before include "Unrefined" , "not yet ready to eat" , "Uncooked or crude" , "not manufactured" , "Naked" .)