Stan messes about in test (10)
I believe the answer is:
'test' is the definition.
(an assessment is a test or examination)
'stan messes about' is the wordplay.
'about' indicates anagramming the letters.
'stanmesses' anagrammed gives 'ASSESSMENT'.
'in' acts as a link.
(Other definitions for assessment that I've seen before include "rating" , "Process of valuation" , "Opinion" , "test" , "Survey" .)