Chief is protected by Sergeant-Major in breach (6)
I believe the answer is:
schism
'breach' is the definition.
(I've seen this in another clue)
'chief is protected by sergeant-major' is the wordplay.
'chief' becomes 'ch' (abbreviation).
'protected by' is an insertion indicator (inserted letters are held or protected).
'sergeant major' becomes 'SM' (abbreviation for sergeant major).
'ch'+'is'='chis'
'chis' going into 'sm' is 'SCHISM'.
'in' is the link.
(Other definitions for schism that I've seen before include "Breach within a church organisation" , "Deep division" , "Formal split in a church" , "Split, often ecclesiastical" , "break" .)