Numb, thrash wastrel (8)
I believe the answer is:
deadbeat
'wastrel' is the definition.
(both can mean a lazy or good-for-nothing person)
'numb thrash' is the wordplay.
'numb' becomes 'dead' (eg a dead arm).
'thrash' becomes 'beat' (synonyms).
'dead'+'beat'='DEADBEAT'
(Other definitions for deadbeat that I've seen before include "idler" , "Lazy, useless person" , "Exhausted - layabout" , "extremely tired" , "idle so-and-so" .)