One who evaluates beasts of burden? So right! (8)
I believe the answer is:
assessor
'one who evaluates' is the definition.
(I've seen this before)
'beasts of burden? so right' is the wordplay.
'beasts of burden?' becomes 'asses' (I've seen this before).
'right' becomes 'r' (common abbreviation).
'asses'+'so'+'r'='ASSESSOR'
(Other definitions for assessor that I've seen before include "Person who tests" , "One who estimates value etc for insurance company" , "Official who evaluates property" , "Examiner" , "Person who estimates values" .)