Keep marine out (6)
I believe the answer is:
remain
'keep' is the definition.
(I know that keep is a more specific form of the action remain)
'marine out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'marine' is an anagram of 'REMAIN'.
(Other definitions for remain that I've seen before include "Stay (in position)" , "Continue in position" , "Survive" , "Last" , "Stay on when others have left" .)