Fluid including oil drained next (9)
I believe the answer is:
following
'next' is the definition.
(synonyms)
'fluid including oil drained' is the wordplay.
'fluid' becomes 'flowing' (synonyms).
'including' means one lot of letters goes inside another.
'drained' means to remove the middle letters.
'oil' with its centre removed is 'ol'.
'flowing' placed around 'ol' is 'FOLLOWING'.
(Other definitions for following that I've seen before include "about to be mentioned" , "fans?" , "Next after" , "Body of supporters" , "Coming after" .)