Hold pin out for an aquatic mammal (7)
I believe the answer is:
dolphin
'for an aquatic mammal' is the definition.
(dolphin is a kind of aquatic mammal)
'hold pin out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'hold'+'pin'='holdpin'
'holdpin' anagrammed gives 'DOLPHIN'.
(Other definitions for dolphin that I've seen before include "Relative of whale" , "Snall type of whale" , "Small sociable whale" , "Sea mammal like large porpoise, friendly towards people" , "Porpoise-like fish" .)