Refueling ship (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">refueling ship</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for oiler that I've seen before include "it may be 4 [SHIPSHAPE]" , "Fuel tanker" , "Grease monkey" , "Petroleum tanker" , "Device that lubricates" .)