In a little Nissan, Guinevere is cheerful (8)
I believe the answer is:
sanguine
'cheerful' is the definition.
(I've seen this before)
'in a little nissan guinevere' is the wordplay.
'in' says the answer is hidden in the clue.
'SANGUINE' can be found hidden inside 'little nissan guinevere'.
'is' acts as a link.
(Other definitions for sanguine that I've seen before include "Bloody - cheerful" , "Cheerful, confident and full of hope" , "Assured, optimistic" , "Optimistic or ruddy" , "Confidently optimistic and cheerful" .)