Rating fools with wrong legend (9)
I believe the answer is:
assessing
'rating' is the definition.
(thesaurus)
'fools with wrong legend' is the wordplay.
'fools' becomes 'asses' (ass can mean a foolish person).
'with' means one lot of letters go next to another.
'wrong' becomes 'sin' (wrong can mean a sin or crime).
'legend' becomes 'g' (leg-end - end letter of 'leg').
'asses'+'sin'+'g'='ASSESSING'
(Other definitions for assessing that I've seen before include "Rating, for taxation say" , "Appraising, evaluating" , "Estimating the worth of" .)