Hot oil pool flowing by island riff-raff (3,6)
I believe the answer is:
hoi polloi
'riff-raff' is the definition.
(I've seen this before)
'hot oil pool flowing by island' is the wordplay.
'hot' becomes 'h' (abbreviation, eg on taps).
'flowing' is an anagram indicator.
'by' is a charade indicator (letters next to each other).
'island' becomes 'i' (geographical abbreviation).
'oil'+'pool'='oilpool'
'oilpool' anagrammed gives 'oipollo'.
'h'+'oipollo'+'i'='HOI-POLLOI'
(Other definitions for hoi polloi that I've seen before include "vulgar rabble" , "The unwashed masses!" , "Plebs" , "commoners" , "Proletariat" .)