Keep marine conditions (6)
I believe the answer is:
remain
'keep' is the definition.
(thesaurus)
'marine conditions' is the wordplay.
'conditions' indicates anagramming the letters (letters must be in a new condition).
'marine' is an anagram of 'REMAIN'.
(Other definitions for remain that I've seen before include "Stay around" , "Stay or continue to be" , "Stay on when others have left" , "Last" , "Continue in position" .)