Nun chose to undergo training as one without equal (8)
I believe the answer is:
nonesuch
'one without equal' is the definition.
(I've seen this in another clue)
'nun chose to undergo training' is the wordplay.
'to undergo training' indicates an anagram (I've seen 'undergoes training' mean this (letters trained or shaped into a new form)).
'nun'+'chose'='nunchose'
'nunchose' with letters rearranged gives 'NONESUCH'.
'as' is the link.
(Other definitions for nonesuch that I've seen before include "a thing unequalled in olden days" , "Matchless thing" , "Perfect example" , "Nonpareil" , "unparalleled" .)