One who examines classes so rigidly boxed (8)
I believe the answer is:
assessor
'one who examines' is the definition.
'classes so rigidly boxed' is the wordplay.
'boxed' says the answer is hidden in the clue (box can mean to contain).
'ASSESSOR' is hidden within 'classes so rigidly'.
(Other definitions for assessor that I've seen before include "Legal adviser" , "Insurance valuer" , "One evaluates property" , "One who estimates or fixes value" , "Maker of valuations" .)