Innkeeper treats oil there (8)
I believe the answer is:
hotelier
'innkeeper' is the definition.
(I've seen this before)
'treats oil there' is the wordplay.
'treats' indicates an anagram.
'oil'+'there'='oilthere'
'oilthere' is an anagram of 'HOTELIER'.
(Other definitions for hotelier that I've seen before include "Conrad Hilton, e.g." , "Provider of rooms" , "Fawlty, perhaps" , "Owner of the Savoy, say" , "I can offer you accommodation" .)