Crustaceans cook finally found on stream (5)
I believe the answer is:
krill
'crustaceans' is the definition.
(I've seen this before)
'cook finally found on stream' is the wordplay.
'finally' says to take the final letters.
'found on' means one lot of letters go next to another.
'stream' becomes 'rill' (rill is a kind of stream).
The final letter of 'cook' is 'k'.
'k'+'rill'='KRILL'
(Other definitions for krill that I've seen before include "Very small crustaceans" , "Small shrimp-like creatures eaten by whales" , "Whales' staple diet" , "What baleen whales eat" , "Crustaceans, diet of whales" .)