Disciple's transgression, second to be admitted (5)
I believe the answer is:
simon
'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'.
(Other definitions for simon that I've seen before include "Simple chap" , "Harris and Coveney" , "One of Christ's apostles" , "Simple pieman" , "Apostle Peter's other name" .)