But not exactly oily (4)
I believe the answer is:
only
'but' is the definition.
(I've seen this in another clue)
'not exactly oily' is the wordplay.
'not' becomes 'o' (I've seen this in other clues).
'exactly oily' becomes 'nly' (I can't explain this - if you can you should give a lot more credence to this answer).
'o'+'nly'='ONLY'
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" .)