In Nissan Guinevere is cheerful (8)
I believe the answer is:
sanguine
'cheerful' is the definition.
(I've seen this in another clue)
'in nissan guinevere is' is the wordplay.
'in' indicates a hidden word.
'SANGUINE' can be found hidden inside 'nissan guinevere is'.
(Other definitions for sanguine that I've seen before include "Expecting good results" , "Confidently optimistic and cheerful" , "Optimistic, confident" , "Cheerfully optimistic" , "Hopeful or optimistic" .)