Thin tree chopped up, a number being bad (8)
I believe the answer is:
thirteen
'a number being bad' is the definition.
(unlucky number)
'thin tree chopped up' is the wordplay.
'chopped up' is an anagram indicator.
'thin'+'tree'='thintree'
'thintree' anagrammed gives 'THIRTEEN'.
(Other definitions for thirteen that I've seen before include "hole possibly" , "13" , "ill-omened figure" , "Supper attendees" , "Number at Last Supper" .)