Mass is bishop’s responsibility, I hear (3)
I believe the answer is:
sea
'mass' is the definition.
(thesaurus)
'bishop's responsibility i hear' is the wordplay.
'bishop's responsibility' becomes 'see' (jurisdiction of a bishop, eg the Holy See).
'i hear' indicates a 'sounds like' (homophone) clue.
'see' sounds like 'SEA'.
'is' is the link.
(Other definitions for sea that I've seen before include "The deep" , "E.g. Baltic" , "Lesser ocean" , "one may be pounding the beach" , "Watery expanse" .)