Boxer who sat up after a clobbering (8)
I believe the answer is:
southpaw
'boxer' is the definition.
(term for a left-handed boxer)
'who sat up after a clobbering' is the wordplay.
'after a clobbering' indicates an anagram.
'who'+'sat'+'up'='whosatup'
'whosatup' anagrammed gives 'SOUTHPAW'.
(Other definitions for southpaw that I've seen before include "Left-handed person, especially a boxer" , "A left-handed boxer who leads with the right" , "To wash up (anag.)" , "Unorthodox leader" , "One using left hand" .)