Southern side's energy (5)
I believe the answer is:
steam
'energy' is the definition.
(I've seen this before)
'southern side's' is the wordplay.
'southern' becomes 's' (abbreviation).
'side' becomes 'team' (a side is a sporting team).
's'+'team'='STEAM'
(Other definitions for steam that I've seen before include "Source of power" , "Momentum - boiled water" , "letter opener?" , "Engine driver" , "Hot water suspended in air" .)