Swell of southern impetus (5)
I believe the answer is:
surge
'swell' is the definition.
(I've seen this in another clue)
'southern impetus' is the wordplay.
'southern' becomes 's' (abbreviation).
'impetus' becomes 'urge' (I've seen this in another clue).
's'+'urge'='SURGE'
'of' is the link.
(Other definitions for surge that I've seen before include "Sudden large increase" , "Outpouring" , "significant increase" , "Sudden rush" , "Sudden push forward" .)