Officer posted to secure gear at sea (8)
I believe the answer is:
sergeant
'officer' is the definition.
(sergeant is a kind of officer)
'posted to secure gear at sea' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post'**).
'to secure' indicates putting letters inside.
'at sea' indicates an anagram.
'gear' anagrammed gives 'rgea'.
'sent' going around 'rgea' is 'SERGEANT'.
(Other definitions for sergeant that I've seen before include "Estrange (anag)" , "Reagents (anag)" , "Soldier and lover in Hardy novel" , "Copper" , "Troy" .)