Petty officer on a merchant ship (5)
I believe the answer is:
bosun
'petty officer on a merchant ship' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bosun that I've seen before include "A short boatswain" , "Crew's foreman" , "Man on board" , "who controls crew?" , "Naval officer" .)