Young at heart learn about fairytale (6)
I believe the answer is:
unreal
'fairytale' is the definition.
(fairytale can mean unreal or imaginary)
'young at heart learn about' is the wordplay.
'at heart' says to take the centre.
'about' indicates an anagram.
The middle of 'young' is 'u'.
'learn' with letters rearranged gives 'nreal'.
'u'+'nreal'='UNREAL'
(Other definitions for unreal that I've seen before include "Artificial, insubstantial" , "Illusory, not actual" , "Not actually existing" , "Lacking material form" , "Not genuine" .)