Not bad, we may become pessimistic (8)
I believe the answer is:
downbeat
'pessimistic' is the definition.
(thesaurus)
'not bad we may become' is the wordplay.
'may become' is an anagram indicator.
'not'+'bad'+'we'='notbadwe'
'notbadwe' with letters rearranged gives 'DOWNBEAT'.
(Other definitions for downbeat that I've seen before include "Pessimistic; understated" , "Depressed - relaxed" , "Casual" , "Gloomy; laid-back" , "Understated; gloomy" .)