Am captivated by newly-made Saint’s endurance (7)
I believe the answer is:
stamina
'endurance' is the definition.
(stamina is a kind of endurance)
'am captivated by newly-made saint's' is the wordplay.
'captivated by' means one lot of letters goes inside another.
'newly made' indicates anagramming the letters.
'saint' with letters rearranged gives 'stina'.
'am' put within 'stina' is 'STAMINA'.
(Other definitions for stamina that I've seen before include "Ability to keep going" , "Endurance - staying power" , "Energy and staying power" , "Staying-power, endurance" , "capability to keep going" .)