Endurance of maiden protected by a saint when converted (7)
I believe the answer is:
stamina
'endurance' is the definition.
(stamina is a kind of endurance)
'maiden protected by a saint when converted' is the wordplay.
'maiden' becomes 'm' (cricket abbreviation).
'protected by' is an insertion indicator.
'when converted' is an anagram indicator (I've seen this in other clues).
'a'+'saint'='asaint'
'asaint' with letters rearranged gives 'staina'.
'm' inserted within 'staina' is 'STAMINA'.
'of' is the link.
(Other definitions for stamina that I've seen before include "Vigour" , "Energy and staying power" , "Indefatigability" , "Enduring energy" , "Ability for sustained effort" .)