Evaluation of agreement covering awful mess (10)
I believe the answer is:
assessment
'evaluation' is the definition.
(thesaurus)
'agreement covering awful mess' is the wordplay.
'agreement' becomes 'assent' (assent is a kind of agreement**).
'covering' means one lot of letters goes inside another.
'awful' is an anagram indicator.
'mess' anagrammed gives 'ssme'.
'assent' enclosing 'ssme' is 'ASSESSMENT'.
'of' is the link.
(Other definitions for assessment that I've seen before include "Evaluation, estimation" , "Appraisal" , "amount to be paid" , "Judgement" , "rating" .)