Go over southern vessel (4)
I believe the answer is:
span
'go over' is the definition.
(I've seen this in another clue)
'southern vessel' is the wordplay.
'southern' becomes 's' (abbreviation).
'vessel' becomes 'pan' (vessel or container for cooking).
's'+'pan'='SPAN'
(Other definitions for span that I've seen before include "Interval; pair of yoked oxen" , "Extent of 20" , "Pair of horses" , "Ox team; bridge" , "Complete duration" .)