But not exactly oily (4)
I believe the answer is:
only
'but' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for only that I've seen before include "Single, solitary" , ". . . . the Lonely (sung by Roy Orbison)" , "Except that; single" , "Mere" , "Solely, lone" .)