What could make Carol happy is questionable (10)
I believe the answer is:
apocryphal
'questionable' is the definition.
(similar in meaning)
'what could make carol happy' is the wordplay.
'what could make' indicates anagramming the letters.
'carol'+'happy'='carolhappy'
'carolhappy' anagrammed gives 'APOCRYPHAL'.
'is' acts as a link.
(Other definitions for apocryphal that I've seen before include "probably made-up" , "Fabulous" .)