A marine's bearing weapons (4)
I believe the answer is:
arms
'weapons' is the definition.
('arm' can be a synonym of 'weapon')
'a marine's bearing' is the wordplay.
'marine' becomes 'rm'.
'bearing' becomes 's'.
'a'+'rm'+'s'='ARMS'
(Other definitions for arms that I've seen before include ". . . . and the Man (G B Shaw) (4)" , "Crest - guns" , "They're used for picking up" , "Ordnance" , "Guns et cetera" .)