Don keeps Phil off this creature (7)
I believe the answer is:
dolphin
'this creature' is the definition.
(dolphins are sea creatures)
'don keeps phil off' is the wordplay.
'keeps' is an insertion indicator.
'off' indicates an anagram.
'phil' with letters rearranged gives 'lphi'.
'don' placed around 'lphi' is 'DOLPHIN'.
(Other definitions for dolphin that I've seen before include "Snall type of whale" , "member in school" , "Sea creature" , "swimmer" , "Sea mammal like large porpoise, friendly towards people" .)