Father's breaking fast in pope's office (6)
I believe the answer is:
papacy
'pope's office' is the definition.
(I've seen this before)
'father's breaking fast' is the wordplay.
'father' becomes 'pa' (pa is a kind of father).
'breaking' indicates putting letters inside.
'fast' becomes 'pacy' (I've seen this before).
'pa' placed into 'pacy' is 'PAPACY'.
'in' is the link.
(Other definitions for papacy that I've seen before include "RC leader's office" , "Church office" , "Vatican leader's office" , "Office of the pontiff" , "high church job" .)