Refueling ship (5)
I believe the answer is:
'refueling ship' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oiler that I've seen before include "it may be 4 [SHIPSHAPE]" , "Fuel tanker" , "Grease monkey" , "Petroleum tanker" , "Device that lubricates" .)