Breakaway group accepting army corps confidence (6)
I believe the answer is:
secret
'confidence' is the definition.
(I've seen this before)
'breakaway group accepting army corps' is the wordplay.
'breakaway group' becomes 'sect' (I've seen this before).
'accepting' indicates putting letters inside (inserted letters are accepted).
'army corps' becomes 'RE' (Royal Engineers).
'sect' placed around 're' is 'SECRET'.
(Other definitions for secret that I've seen before include "Confidential or mysterious" , "away from scrutiny" , "'Hidden, covert (6)'" , "Confidential, not to be revealed" , "Not generally known" .)