Keep marine working (6)
I believe the answer is:
remain
'keep' is the definition.
(thesaurus)
'marine working' is the wordplay.
'working' indicates an anagram.
'marine' with letters rearranged gives 'REMAIN'.
(Other definitions for remain that I've seen before include "Stay (in a place)" , "Continue in position" , "Last" , "Airmen (anag.)" , "Don't leave (the EU?)" .)