Is demoting a sergeant such punishment? (8)
I believe the answer is:
corporal
'punishment?' is the definition.
(corporal punishment is a kind of punishment)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for corporal that I've seen before include "NCO rank below sergeant" , "One trained for engagement" , "Of the body, or army NCO" , "Poor Carl (anag)" , "material" .)