Nearly learned to inject cocaine and heroin (4)

'heroin' is the definition.
(informal term for heroin)

'nearly learned to inject cocaine' is the wordplay.
'nearly' means to remove the last letter.
'learned' becomes 'sage' (both can mean wise).
'to inject' indicates putting letters inside (inserted letters are injected).
'cocaine' becomes 'C' (drugs abbreviation).
'sage' with its last letter removed is 'sag'.
'sag' enclosing 'c' is 'SCAG'.

'and' acts as a link.