Joan's fanciful hero (5)
I believe the answer is:
jason
'hero' is the definition.
(I've seen this before)
'joan's fanciful' is the wordplay.
'fanciful' indicates anagramming the letters (a fanciful or silly ordering of the letters).
'joans' with letters rearranged gives 'JASON'.
(Other definitions for jason that I've seen before include "Boy" , "He led the Argonauts to find the Golden Fleece" , "Leader of the Argonauts who sailed in quest of the Golden Fleece" , "Greek hero, captain of the Argo" , "Argonaut captain" .)