Fools small people with essentially untrustworthy evaluation (10)
I believe the answer is:
assessment
'evaluation' is the definition.
(I've seen this before)
'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' means one lot of letters go next to another.
'essentially' means to look at the middle letters.
The middle 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" .)