Everyman's among fools, ultimately preposterous in prevalent estimation (10)
I believe the answer is:
assessment
'estimation' is the definition.
(I've seen this before)
'everyman's among fools ultimately preposterous in prevalent' is the wordplay.
'everyman' becomes 'me' (where the crossword setter is 'Everyman').
'among' indicates putting letters inside (inserted letters are placed among others).
'fools' becomes 'asses' (ass can mean a foolish person).
'ultimately' says to take the final letters.
'in prevalent' becomes 'nt' (I can't explain this - if you can you should give a lot more credence to this answer).
The final letter of 'preposterous' is 's'.
'asses'+'s'+'nt'='assessnt'
'me' inserted inside 'assessnt' is 'ASSESSMENT'.
Can you help me to learn more?
(Other definitions for assessment that I've seen before include "test" , "amount to be paid" , "Examination" , "Appraisal" , "Review" .)