It's a man possibly revealing staying power (7)
I believe the answer is:
stamina
'staying power' is the definition.
(synonyms)
'it's a man possibly' is the wordplay.
'possibly' indicates an anagram.
'its'+'a'+'man'='itsaman'
'itsaman' anagrammed gives 'STAMINA'.
'revealing' is the link.
(Other definitions for stamina that I've seen before include "Vigour" , "The ability to sustain prolonged physical or mental effort" , "capability to keep going" , "Ability to keep going" , "Resilience" .)