Judge very rigorous at first on fools (8)
I believe the answer is:
assessor
'judge' is the definition.
(I've seen this before)
'very rigorous at first on fools' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'at first' says to take the initial letters.
'on' says to put letters next to each other.
'fools' becomes 'asses' (ass can mean a foolish person).
The first letter of 'rigorous' is 'r'.
'so'+'r'='sor'
'sor' after 'asses' is 'ASSESSOR'.
(Other definitions for assessor that I've seen before include "Valuer, estimator" , "Person providing marks" , "property valuer" , "One values property" , "Insurance valuer" .)