Incentive to turn up in southern river (4)
I believe the answer is:
spur
'incentive' is the definition.
(an incentive spurs someone on)
'turn up in southern river' is the wordplay.
'turn' says the letters should be written backwards.
'in' means one lot of letters goes inside another.
'southern' becomes 's' (abbreviation).
'river' becomes 'r' (abbreviation for river).
'up' in reverse letter order is 'pu'.
's'+'r'='sr'
'pu' inserted into 'sr' is 'SPUR'.
'to' is the link.
(Other definitions for spur that I've seen before include "Hurry up" , "Goad as for a horse" , "slip road" , "Goad - branch line" , "Rider's heel wheel" .)