Southern plonk promotion: 'It ignites!' (5,4)
I believe the answer is:
spark plug
'it ignites' is the definition.
'southern plonk promotion' is the wordplay.
'southern' becomes 's' (abbreviation).
'plonk' becomes 'park'.
'promotion' becomes 'plug' (plug is a kind of promotion).
's'+'park'+'plug'='SPARK-PLUG'
(Other definitions for spark plug that I've seen before include "Electrical device in a car's cylinder head" , "starter" , "Ignition device in an internal-combustion engine" , "Does it help to start driving" , "Electrical device fitted in the cylinder heads of an internal-combustion engine" .)