Cooked fat eel and raw, small crustacean (5,4)
I believe the answer is:
water flea
'small crustacean' is the definition.
'water flea' can be an answer for 'crustacean' (water flea is a kind of crustacean). I am not certain of the 'small' bit.
'cooked fat eel and raw' is the wordplay.
'cooked' indicates an anagram (letters cooked into a new form).
'and' says to put letters next to each other.
'fat'+'eel'='fateel'
'fateel' after 'raw' is 'rawfateel'.
'rawfateel' with letters rearranged gives 'WATER FLEA'.
Can you help me to learn more?
(Other definitions for water flea that I've seen before include "Tiny crustacean" , "Tiny creature" , "Common name for Daphnia" .)