Staying power in a man -- it's unusual! (7)
I believe the answer is:
stamina
'staying power' is the definition.
(synonyms)
'a man it's unusual' is the wordplay.
'unusual' indicates an anagram.
'a'+'man'+'its'='amanits'
'amanits' anagrammed gives 'STAMINA'.
'in' acts as a link.
(Other definitions for stamina that I've seen before include "Staying power, ability to withstand prolonged strain" , "Ability to maintain effort" , "Parts of flowers" , "capacity to endure" , "Vigour" .)