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. Perhaps they are linked in a way I don't understand?
'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' (resembles 0 - 'no' can mean 'zero') and 'o' is located in the answer.
This explanation may well be incorrect...
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" .)