Poor Carl walloped an NCO (8)
I believe the answer is:
corporal
'an nco' is the definition.
(I've seen this before)
'poor carl walloped' is the wordplay.
'walloped' is an anagram indicator (I've seen 'wallop' mean this).
'poor'+'carl'='poorcarl'
'poorcarl' is an anagram of 'CORPORAL'.
(Other definitions for corporal that I've seen before include "Petty officer" , "Of the body, like NCO" , "NCO next under a sergeant" , "Army rank" , "Like old-fashioned punishment" .)