Marine creature in hold at sea retains power (7)
I believe the answer is:
dolphin
'marine creature' is the definition.
(type of marine animal)
'in hold at sea retains power' is the wordplay.
'at sea' indicates anagramming the letters.
'retains' means one lot of letters goes inside another.
'power' becomes 'p'.
'in'+'hold'='inhold'
'inhold' with letters rearranged gives 'dolhin'.
'dolhin' going around 'p' is 'DOLPHIN'.
(Other definitions for dolphin that I've seen before include "member in school" , "Grampus" , "Small toothed whale, larger than a porpoise" , "Sociable sea mammal, sacred in ancient Greece" , "Marine mammal with an elongated beak-like head" .)