Substance (heroin) found in part of Ireland (5)
I believe the answer is:
meath
'part of ireland' is the definition.
(Meath is a county of Ireland)
'substance heroin' is the wordplay.
'substance' becomes 'meat' (meat is a kind of substance).
'heroin' becomes 'H' (H is a slang term for heroin).
'meat'+'h'='MEATH'
'found in' is the link.
(Other definitions for meath that I've seen before include "part of Ireland" , "Irish county of ancient kings, site of Tara" , "Tara's county" , "County NW of Dublin" , "lost province" .)