Fools before singular judge (6)
I believe the answer is:
assess
'judge' is the definition.
(to assess is to judge or rate)
'fools before singular' is the wordplay.
'fools' becomes 'asses' (ass can mean a foolish person).
'before' means one lot of letters go next to another.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'asses'+'s'='ASSESS'
(Other definitions for assess that I've seen before include "Remedy for all ills" , "Place a value on" , "Fix the amount of tax" , "Gauge, evaluate" , "Reckon value" .)