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