Keep marine moving (6)
I believe the answer is:
remain
'keep' is the definition.
(thesaurus)
'marine moving' is the wordplay.
'moving' indicates an anagram (letters are moved around).
'marine' anagrammed gives 'REMAIN'.
(Other definitions for remain that I've seen before include "Don't leave (the EU?)" , "Stay (in position)" , "Stay on after others have gone" , "Stay behind" , "Airmen (anag.)" .)