Left with old flame after party in Northern Ireland held in apartment (6)
I believe the answer is:
duplex
'apartment' is the definition.
(I know that duplex is a type of apartment)
'left with old flame after party in northern ireland' is the wordplay.
'left' becomes 'l' (common abbreviation).
'with' is a charade indicator (letters next to each other).
'old flame' becomes 'ex' (an ex-partner).
'after' is a charade indicator (letters next to each other).
'party in northern ireland' becomes 'DUP' (Democratic Unionist Party).
'l' next to 'ex' is 'lex'.
'lex' after 'dup' is 'DUPLEX'.
'held in' acts as a link.
(Other definitions for duplex that I've seen before include "A kind of flat on two floors" , "Apartment on two floors" , "Having two parts, perhaps house" , "House with two units sharing a common wall" , "Two-fold" .)