Needle for a right idiot on heroin (6)
I believe the answer is:
harass
'needle' is the definition.
(thesaurus)
'a right idiot on heroin' is the wordplay.
'right' becomes 'r' (common abbreviation).
'idiot' becomes 'ass' (I've seen this before**).
'on' means one lot of letters go next to another.
'heroin' becomes 'h' ('H' can be a synonym of 'heroin').
'a'+'r'+'ass'='arass'
'arass' after 'h' is 'HARASS'.
'for' acts as a link.
(Other definitions for harass that I've seen before include "bug" , "Nag" , "Molest, annoy" , "Constantly intimidate" , "Annoy or badger constantly" .)