Agree to cover up unfortunate mess in appraisal (10)
I believe the answer is:
assessment
'appraisal' is the definition.
('assessment' can be a synonym of 'appraisal')
'agree to cover up unfortunate mess' is the wordplay.
'agree to' becomes 'assent' (I've seen this before).
'cover' is an insertion indicator.
'up unfortunate mess' becomes 'ssme' (I am not sure about this - if you are sure you should believe this answer much more).
'assent' placed around 'ssme' is 'ASSESSMENT'.
'in' is the link.
(Other definitions for assessment that I've seen before include "Judgement" , "test" , "Appraisal" , "Survey" , "Opinion" .)