One reeks after changing oil (8)
I believe the answer is:
kerosene
'oil' is the definition.
(thesaurus)
'one reeks after changing' is the wordplay.
'after' means one lot of letters go next to another.
'changing' indicates anagramming the letters.
'one'+'reeks'='onereeks'
'onereeks' with letters rearranged gives 'KEROSENE'.
(Other definitions for kerosene that I've seen before include "Paraffin oil (E, not I)" , "Oil used as fuel in lamps and heaters" , "Sore knee (anag.)" , "No seeker for American fuel oil" , "Combustible" .)