Fools start to suspect judge (6)
I believe the answer is:
assess
'judge' is the definition.
(to assess is to judge or rate)
'fools start to suspect' is the wordplay.
'fools' becomes 'asses' (ass can mean a foolish person).
'start to' says to take the initial letters.
The initial letter of 'suspect' is 's'.
'asses'+'s'='ASSESS'
(Other definitions for assess that I've seen before include "Evaluate worth" , "Place a value on" , "Judge the worth of something" , "Estimate the worth or quality of" , "Fix the amount of tax" .)