Nothing is not all bad (6)
I believe the answer is:
naught
'nothing' is the definition.
(synonyms)
'not all bad' is the wordplay.
'not all' means to remove the last letter (not quite all of it).
'bad' becomes 'naughty' (naughty means badly behaved).
'naughty' with its final letter taken away is 'NAUGHT'.
'is' is the link.
(Other definitions for naught that I've seen before include "[NOT A WHIT]" , "The figure O" , "bad" , "Come to --; set at --" , "Old word for nothing, sounds like zero" .)