Most reliable mate finally breaking confidence (6)
I believe the answer is:
truest
'most reliable' is the definition.
('true' can be similar in meaning to 'reliable')
'mate finally breaking confidence' is the wordplay.
'mate finally' becomes 'e' (I can't explain this - if you can you should believe this answer much more).
'breaking' means one lot of letters goes inside another.
'confidence' becomes 'trust' (synonyms).
'e' going within 'trust' is 'TRUEST'.
Can you help me to learn more?
(Other definitions for truest that I've seen before include "Utters in most straight way" , "Most reliable" , "Straightest and least false" , "Most loyal or veracious" , "Superlatively sincere" .)