Sea creature exercise pin-hold (7)
I believe the answer is:
dolphin
'sea creature' is the definition.
(I've seen this before)
'exercise pin-hold' is the wordplay.
'exercise' indicates an anagram (I've seen 'exercising' mean this (letters exercise or move around)).
'pin'+'hold'='pinhold'
'pinhold' anagrammed gives 'DOLPHIN'.
(Other definitions for dolphin that I've seen before include "swimmer" , "Snall type of whale" , "Bottlenose creature?" , "Porpoise-like fish" , "member in school" .)