One of those in the abbey needing no introduction leads monks and nuns and one of the flock (6)
I believe the answer is:
layman
'one of those in the abbey needing no introduction' is the definition.
I can't tell whether this defines the answer.
'leads monks and nuns and one of the flock' is the wordplay.
'leads' suggests taking the first letters (I've seen 'leading' mean this).
'and' says to put letters next to each other.
'one of the flock' becomes 'lay' (I can't justify this - if you can you should give a lot more credence to this answer).
The initial letters of 'monks and nuns' is 'man'.
'man' put after 'lay' is 'LAYMAN'.
Can you help me to learn more?
(Other definitions for layman that I've seen before include "Non-expert" , "member of the congregation" , "He's not a priest or specialist" , "Non-ordained church member" , "He's not a clergyman" .)