Drug is one of five in the East (7)
I believe the answer is:
quinine
'drug' is the definition.
(I've seen this before)
'one of five in the east' is the wordplay.
'one of five' becomes 'quin' (I've seen this before).
'the east' becomes 'e' ('E' can be a synonym of 'east').
'quin'+'in'+'e'='QUININE'
'is' acts as a link.
(Other definitions for quinine that I've seen before include "Bitter substance from cinchona bark" , "Bitter stuff" , "Anti-malarial bark extract" , "Tonic flavouring" , "Medicinal extract from cinchona bark" .)