Staying power of a man -- it's staggering (7)
I believe the answer is:
stamina
'staying power' is the definition.
(synonyms)
'a man it's staggering' is the wordplay.
'staggering' indicates anagramming the letters (letters stagger around).
'a'+'man'+'its'='amanits'
'amanits' is an anagram of 'STAMINA'.
'of' acts as a link.
(Other definitions for stamina that I've seen before include "Ability to maintain effort" , "Parts of flowers" , "Endurance, strength to stay the course" , "grit" , "Enduring energy" .)