Against having party in flat (5)
I believe the answer is:
condo
'flat' is the definition.
(short for condominium, a type of flat in the US)
'against having party' is the wordplay.
'against' becomes 'con' (I've seen this before).
'having' says to put letters next to each other.
'party' becomes 'do' (do is a kind of party).
'con'+'do'='CONDO'
'in' is the link.
(Other definitions for condo that I've seen before include "accommodation out west" , "American apartment" , "flat" , "apartment block in Toronto" , "Apartment in North America" .)