Judge fools head of state (6)
I believe the answer is:
assess
'judge' is the definition.
(to assess is to judge or rate)
'fools head of state' is the wordplay.
'fools' becomes 'asses' (ass can mean a foolish person).
'head of' says to take the initial letters.
The first letter of 'state' is 's'.
'asses'+'s'='ASSESS'
(Other definitions for assess that I've seen before include "Evaluate worth" , "price" , "(Of a tax) determine the amount to be paid" , "Estimate value (for tax)" , "'Evaluate, appraise (6)'" .)