Petty officer on sub at sea (5)
I believe the answer is:
bosun
'petty officer' is the definition.
(I've seen this before)
'on sub at sea' is the wordplay.
'at sea' indicates an anagram.
'on'+'sub'='onsub'
'onsub' anagrammed gives 'BOSUN'.
(Other definitions for bosun that I've seen before include "chair for some work on the side" , "An abbreviated boatswain" , "Naval officer" , "Crew's foreman" , "Officer on board" .)