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' means one lot of letters goes inside another (inserted letters are held or protected).
'sergeant major' becomes 'SM' (abbreviation for sergeant major).
'ch'+'is'='chis'
'chis' put within '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" .)