Drenches Southern trees (5)
I believe the answer is:
soaks
'drenches' is the definition.
('soak' can be a synonym of 'drench')
'southern trees' is the wordplay.
'southern' becomes 's' (abbreviation).
'trees' becomes 'oaks' (oak is a kind of tree).
's'+'oaks'='SOAKS'
(Other definitions for soaks that I've seen before include "big downers" , "Drenches" , "Saturates" .)