Push heroin in confidence (6)
I believe the answer is:
thrust
'push' is the definition.
(synonyms)
'heroin in confidence' is the wordplay.
'heroin' becomes 'H' (synonyms).
'in' means one lot of letters goes inside another.
'confidence' becomes 'trust' (synonyms).
'h' placed inside 'trust' is 'THRUST'.
(Other definitions for thrust that I've seen before include "Shove forward" , "Pierce" , "Push forward with force - gist" , "press" , "Violently push -- gist" .)