Keep marine working (6)
I believe the answer is:
remain
'keep' is the definition.
(thesaurus)
'marine working' is the wordplay.
'working' indicates anagramming the letters.
'marine' is an anagram of '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?)" .)