Southern union about to reach agreement (6)
I believe the answer is:
unison
'agreement' is the definition.
(I know that unison is a type of agreement)
'southern union about' is the wordplay.
'southern' becomes 's' (abbreviation).
'about' indicates putting letters inside.
's' inserted inside 'union' is 'UNISON'.
'to reach' is the link.
(Other definitions for unison that I've seen before include "using same pitch" , "All voices together" , "As one voice" , "Trade union (where all agree?)" , "As one, singers perhaps" .)