Remorseful prisoner — first one in the religious ceremony (8)
I believe the answer is:
contrite
'remorseful' is the definition.
(synonyms)
'prisoner first one in the religious ceremony' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'first one in' indicates taking the first letters.
'religious ceremony' becomes 'rite' (rite is a kind of religious ceremony).
The first letter of 'the' is 't'.
'con'+'t'+'rite'='CONTRITE'
(Other definitions for contrite that I've seen before include "in a sorry state indeed" , "Feeling remorse" , "Regretful" , "Repentant, remorseful" , "Penitent, feeling guilty" .)