Don't be so optimistic — it's no good (8)
I believe the answer is:
hopeless
'good' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can't see?
'don't be so optimistic it's no' is the wordplay.
I cannot quite understand how this works, but
'it's' could be 'es' ('e' can mean 'electronic' which is similar to 'IT') and 'es' is located in the answer.
'no' could be 'o' (O resembles 0) and 'o' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for hopeless that I've seen before include "Having no reason to expect success" , "Despairing or futile" , "Incompetent" , "Feeling or causing despair" , "With no chance of success" .)