Sailor's flag at sea (4)
I believe the answer is:
jack
I believe this clue is a double definition.
'sailor's' is the first definition.
(informal term for a sailor)
'flag at sea' is the second definition.
(jack can mean a flag flown on a ship)
(Other definitions for jack that I've seen before include "Jill's companion" , "Man in navy" , "Lifting device - flag - boy's name - card - fish" , "Knave (cards)" , "Dog akin to 9" .)