Papacy's unknown saint beginning to engage in breach (4,3)
I believe the answer is:
holy see
'papacy's' is the definition.
(I have seen 'The papacy ' mean 'holy see' so perhaps 'papacy' could also mean 'holy see')
'unknown saint beginning to engage in breach' is the wordplay.
'unknown' becomes 'y' (algebra).
'saint' becomes 's'.
'beginning to' suggests taking the first letters.
'in' is an insertion indicator.
'breach' becomes 'hole' (I am not sure about this - if you are sure you should believe this answer much more).
The first letter of 'engage' is 'e'.
'y'+'s'+'e'='yse'
'yse' going into 'hole' is 'HOLY SEE'.
Can you help me to learn more?
(Other definitions for holy see that I've seen before include "Rome ecclesiastically" , "Church court" , "Diocese of Rome" , "Pope's bishopric" , "palace in Rome" .)