Drug supplied by ambassador meeting politician (4)
I believe the answer is:
hemp
'drug' is the definition.
(I've seen this before)
'ambassador meeting politician' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'meeting' means one lot of letters go next to another.
'politician' becomes 'mp' (Member of Parliament).
'he'+'mp'='HEMP'
'supplied by' acts as a link.
(Other definitions for hemp that I've seen before include "Plant fibre used to make rope" , "tea" , "Grass" , "Marijuana" , "Plant for rope or pot" .)