Display stamina with fit one running (13)
I believe the answer is:
manifestation
'display' is the definition.
(I've seen this in another clue)
'stamina with fit one running' is the wordplay.
'with' is a charade indicator (letters next to each other).
'running' indicates an anagram.
'fit'+'one'='fitone'
'stamina' put after 'fitone' is 'fitonestamina'.
'fitonestamina' anagrammed gives 'MANIFESTATION'.
(Other definitions for manifestation that I've seen before include "Ghostly appearance" , "Sign" , "Display" , "Appearance of a ghost" , "Demonstration" .)