Developed sore knee getting fuel (8)
I believe the answer is:
kerosene
'fuel' is the definition.
(kerosene is a kind of fuel)
'developed sore knee getting' is the wordplay.
'developed' is an anagram indicator.
'getting' is a charade indicator (letters next to each other).
'sore'+'knee'='soreknee'
'soreknee' is an anagram of 'KEROSENE'.
(Other definitions for kerosene that I've seen before include "Combustible" , "Sore knee (anag.)" , "No seeker for American fuel oil" , "Oil used as fuel in lamps and heaters" , "Paraffin oil (E, not I)" .)