Stage complete? It's a myth (6)
I believe the answer is:
legend
'a myth' is the definition.
(thesaurus)
'stage complete?' is the wordplay.
'stage' becomes 'leg' ('leg' can be a synonym of 'stage').
'complete?' becomes 'end' (I've seen this in other clues).
'leg'+'end'='LEGEND'
'it's' acts as a link.
(Other definitions for legend that I've seen before include "Celebrity" , "Traditional story; caption" , "Traditional story or myth" , "Tale of folklore" , "Old tale" .)