Story about noble rider's first challenge (4,5)
I believe the answer is:
tall order
'challenge' is the definition.
(a tall order is a challenging demand)
'story about noble rider's first' is the wordplay.
'story' becomes 'tale' (synonyms).
'about' indicates putting letters inside.
'noble' becomes 'lord' (synonyms).
'rider's first' becomes 'r' (1st letter of 'rider').
'tale' going around 'lord' is 'tallorde'.
'tallorde'+'r'='TALL ORDER'
(Other definitions for tall order that I've seen before include "Formidable task" , "Lot to ask" , "as the building of Babel was?" , "for brotherhood of high standing" , "delivery of sequoias?" .)