Discover southern states, not entirely south (4,3)
I believe the answer is:
suss out
'discover' is the definition.
(I've seen this before)
'southern states not entirely south' is the wordplay.
'southern' becomes 's' (abbreviation).
'states' becomes 'US' (abbreviation for the United States).
'not entirely' means to remove the last letter.
'south' with its last letter taken off is 'sout'.
's'+'us'+'sout'='SUSS OUT'
(Other definitions for suss out that I've seen before include "Grasp" , "Check into" , "Solve" , "Discover" , "Examine to determine accuracy" .)