Square on which Parisian supplies drug (7)
I believe the answer is:
quinine
'drug' is the definition.
(quinine is a kind of drug)
'square on which parisian' is the wordplay.
'square' becomes 'nine' (nine is a square number).
'on' is a charade indicator (letters next to each other).
'which parisian' becomes 'qui' ('which' in French).
'nine' after 'qui' is 'QUININE'.
'supplies' is the link.
(Other definitions for quinine that I've seen before include "Bitter stuff" , "Alkaloid from tree" , "Bitter compound used as a tonic" , "Substance once used to treat malaria" , "What gives tonic water its taste" .)