Decisively influence southern faction (5)
I believe the answer is:
swing
'decisively influence' is the definition.
(as in swinging an election)
'southern faction' is the wordplay.
'southern' becomes 'S' (abbreviation).
'faction' becomes 'wing'.
's'+'wing'='SWING'
(Other definitions for swing that I've seen before include "Jazz-like dance music" , "Sweeping blow or stroke" , "Piece of playground equipment" , "be hanged?" , "change in voting" .)