Belgrade nun spinning modern folktale (5,6)
I believe the answer is:
urban legend
'modern folktale' is the definition.
(an urban legend is a made-up story)
'belgrade nun spinning' is the wordplay.
'spinning' indicates an anagram.
'belgrade'+'nun'='belgradenun'
'belgradenun' with letters rearranged gives 'URBAN LEGEND'.
(Other definitions for urban legend that I've seen before include "popular myth" , "apocryphal" .)