Works in the abbey occasionally beneath writing rooms (12)
I believe the answer is:
understudies
'works in the abbey occasionally' is the definition.
I can't judge whether this definition defines the answer.
'beneath writing rooms' is the wordplay.
'beneath' becomes 'under' (I've seen this before).
'writing rooms' becomes 'studies' (I can't explain this - if you can you should give a lot more credence to this answer).
'under'+'studies'='UNDERSTUDIES'
Can you help me to learn more?
(Other definitions for understudies that I've seen before include "Substitute actors" , "Standby actors" .)