Fish developed ears to assimilate noise (7)
I believe the answer is:
sardine
'fish' is the definition.
(sardine is a kind of fish)
'developed ears to assimilate noise' is the wordplay.
'developed' is an anagram indicator.
'to assimilate' is an insertion indicator.
'noise' becomes 'din' (din is a kind of noise).
'ears' anagrammed gives 'sare'.
'sare' enclosing 'din' is 'SARDINE'.
(Other definitions for sardine that I've seen before include "Fish (often tinned)" , "Young pilchard" , "A diner's small fish" , "Small pilchard" , "Fish in tin?" .)