Hospital treated polio with oil for masses (3,6)
I believe the answer is:
hoi polloi
'masses' is the definition.
(Greek term for the common people)
'hospital treated polio with oil' is the wordplay.
'hospital' becomes 'h' (abbreviation for hospital on maps).
'treated' is an anagram indicator.
'with' says to put letters next to each other.
'polio' after 'oil' is 'oilpolio'.
'oilpolio' with letters rearranged gives 'oipolloi'.
'h'+'oipolloi'='HOI POLLOI'
'for' acts as a link.
(Other definitions for hoi polloi that I've seen before include "The plain people in Greece" , "commoners" , "The common people - in Greece" , "ordinary people" , "The common people generally" .)