Judgement sent masses haywire (10)
I believe the answer is:
assessment
'judgement' is the definition.
(synonyms)
'sent masses haywire' is the wordplay.
'haywire' indicates anagramming the letters.
'sent'+'masses'='sentmasses'
'sentmasses' is an anagram of 'ASSESSMENT'.
(Other definitions for assessment that I've seen before include "Appraisal" , "Review" , "Evaluation, estimation" , "Process of valuation" , "Opinion" .)