Oil treatment for sore knee (8)
I believe the answer is:
kerosene
'oil' is the definition.
(I've seen this before)
'treatment for sore knee' is the wordplay.
'treatment for' indicates anagramming the letters (I've seen this in other clues).
'sore'+'knee'='soreknee'
'soreknee' anagrammed gives 'KEROSENE'.
(Other definitions for kerosene that I've seen before include "No seeker for American fuel oil" , "Combustible" , "Oil used as fuel in lamps and heaters" , "Paraffin oil (E, not I)" , "Sore knee (anag.)" .)