Idiot on heroin in clergy (5)
I believe the answer is:
cloth
'clergy' is the definition.
(I have seen 'The clergy ' mean 'cloth' so perhaps 'clergy' could also mean 'cloth')
'idiot on heroin' is the wordplay.
'idiot' becomes 'clot' (I've seen this before).
'on' says to put letters next to each other.
'heroin' becomes 'H' (synonyms).
'clot'+'h'='CLOTH'
'in' acts as a link.
(Other definitions for cloth that I've seen before include "Fabric, textile" , "Sails" , "The clergy" , "Woven material" , "Gingham, say" .)