Losing head, one working hard in fuel ship (5)
I believe the answer is:
oiler
'fuel ship' is the definition.
(I've seen this in another clue)
'losing head one working hard' is the wordplay.
'losing head' means to remove the first letter.
'one working hard' becomes 'toiler' (I've seen this in another clue).
'toiler' with its first letter taken away is 'OILER'.
'in' is the link.
(Other definitions for oiler that I've seen before include "If not well" , "Loire (anag.)" , "Petroleum tanker" , "It carries fuel" , "Ship" .)