It's a man running, showing staying power (7)
I believe the answer is:
stamina
'staying power' is the definition.
('stamina' can be a synonym of 'staying power')
'it's a man running' is the wordplay.
'running' indicates an anagram.
'its'+'a'+'man'='itsaman'
'itsaman' anagrammed gives 'STAMINA'.
'showing' is the link.
(Other definitions for stamina that I've seen before include "grit" , "Vigour" , "Ability for sustained effort" , "legs" , "Endurance - staying power" .)