Popular story to conclude below stage (6)
I believe the answer is:
legend
'popular story' is the definition.
'conclude below stage' is the wordplay.
'conclude' becomes 'end' (I've seen this before).
'below' means one lot of letters go next to another.
'stage' becomes 'leg' (synonyms).
'end' after 'leg' is 'LEGEND'.
'to' acts as a link.
(Other definitions for legend that I've seen before include "Old yarn" , "Old tale" , "Traditional story - inscription" , "Myth or map symbol" , "Traditional story or myth" .)