Fools with Southern judge (6)
I believe the answer is:
assess
'judge' is the definition.
(to assess is to judge or rate)
'fools with southern' is the wordplay.
'fools' becomes 'asses' (ass can mean a foolish person).
'with' means one lot of letters go next to another.
'southern' becomes 's' (abbreviation).
'asses'+'s'='ASSESS'
(Other definitions for assess that I've seen before include "Estimate value (for tax)" , "Evaluate worth" , "Judge the quality of" , "Estimate the worth or quality of" , "price" .)