Fools small people with essentially untrustworthy evaluation (10)
I believe the answer is:
assessment
'evaluation' is the definition.
(I've seen this in another clue)
'fools small people with essentially untrustworthy' is the wordplay.
'fools' becomes 'asses' (ass can mean a foolish person).
'small' becomes 's' (abbreviation - e.g. clothes size).
'people' becomes 'men' (men are people).
'with' is a charade indicator (letters next to each other).
'essentially' indicates the central letters.
The middle letter of 'untrustworthy' is 't'.
'asses'+'s'+'men'+'t'='ASSESSMENT'
(Other definitions for assessment that I've seen before include "Examination" , "Appraisal" , "amount to be paid" , "Review" , "Evaluation, estimation" .)