Disciple's transgression, second to be admitted (5)
I believe the answer is:
Here is my best explanation:
'disciple's' is the definition.
(I've seen this before)
'transgression second to be admitted' is the wordplay.
'transgression' becomes 'sin' (sin is a kind of transgression).
'second' becomes 'mo' (both are short periods of time).
'to be admitted' indicates putting letters inside.
'sin' enclosing 'mo' is 'SIMON'.