Confident and with good reason, drinking half of Guinness (8)
I believe the answer is:
sanguine
'confident' is the definition.
(thesaurus)
'with good reason drinking half of guinness' is the wordplay.
'with good reason' becomes 'sane' (I can't justify this - if you can you should give a lot more credence to this answer).
'drinking' indicates putting letters inside.
'half of' means to take half the letters.
'guinness' cut in half is 'guin'.
'sane' enclosing 'guin' is 'SANGUINE'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for sanguine that I've seen before include "Hopeful of winning - or ruddy" , "Hopeful or optimistic" , "Assured, optimistic" , "Confidently optimistic and cheerful" , "Cheerful, confident and full of hope" .)