Southern post office applies for partners (7)
I believe the answer is:
spouses
'for partners' is the definition.
('spouse' can be a synonym of 'partner')
'southern post office applies' is the wordplay.
'southern' becomes 's' (abbreviation).
'post office' becomes 'po' (abbreviation).
'applies' becomes 'uses' ('use' can be a synonym of 'apply').
's'+'po'+'uses'='SPOUSES'
(Other definitions for spouses that I've seen before include "Partners in marriage" , "Wives or husbands" .)