After transgression, chastise heartlessly (5)
I believe the answer is:
since
'after' is the definition.
(since can mean after a certain time)
'transgression chastise heartlessly' is the wordplay.
'transgression' becomes 'sin' (sin is a kind of transgression).
'heartlessly' says to hollow out the word (remove centre letters).
'chastise' with its centre taken out is 'ce'.
'sin'+'ce'='SINCE'
(Other definitions for since that I've seen before include "in the past" , "From that time one" , "Seeing that" , "Because; ago" .)