Introduces offence after addict swallows heroin (6,2)

'introduces' is the definition.
('usher in' can be a synonym of 'introduce')

'offence after addict swallows heroin' is the wordplay.
'offence' becomes 'sin' (a sin is an offence or transgression).
'after' says to put letters next to each other.
'addict' becomes 'user' (drug user).
'swallows' indicates putting letters inside.
'heroin' becomes 'H' (H is a slang term for heroin).
'user' placed around 'h' is 'usher'.
'sin' put after 'usher' is 'USHERS IN'.