Boxer who sat up after crumbling (8)
I believe the answer is:
southpaw
'boxer' is the definition.
(I've seen this before)
'who sat up after crumbling' is the wordplay.
'after' is a charade indicator (letters next to each other).
'crumbling' indicates an anagram.
'who'+'sat'+'up'='whosatup'
'whosatup' is an anagram of 'SOUTHPAW'.
(Other definitions for southpaw that I've seen before include "Boxer who leads with the right hand" , "Boxer leading with right hand" , "Right leader" , "Left-hander" , "Left-handed pugilist" .)