Staying power in a man - it's surprising (7)
I believe the answer is:
stamina
'staying power' is the definition.
('stamina' can be a synonym of 'staying power')
'a man it's surprising' is the wordplay.
'surprising' indicates anagramming the letters.
'a'+'man'+'its'='amanits'
'amanits' with letters rearranged gives 'STAMINA'.
'in' is the link.
(Other definitions for stamina that I've seen before include "grit" , "fortitude" , "Endurance - staying power" , "perseverance" , "Staying power, ability to withstand prolonged strain" .)