Plot to get drug for old monk (4)
I believe the answer is:
bede
'old monk' is the definition.
(I've seen this before)
'plot to get drug' is the wordplay.
'plot' becomes 'bed' (a plot of land).
'to get' is a charade indicator (letters next to each other).
'drug' becomes 'e' (E is another name for ecstasy).
'bed'+'e'='BEDE'
'for' is the link.
(Other definitions for bede that I've seen before include "Venerable English saint" , "Venerable English historian, d. 735" , "Venerable author of The Ecclesiastical History of the English People, d. 735" , "Early English historian" , "venerable person?" .)