Confidentially intimate feeling in functions with Boris, ultimately (8)
I believe the answer is:
cosiness
'confidentially intimate feeling' is the definition.
(feeling of secret intimacy)
'functions with boris ultimately' is the wordplay.
'functions' becomes 'cosines' (cosine is a kind of function).
'with' is a charade indicator (letters next to each other).
'boris ultimately' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'cosines'+'s'='COSINESS'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for cosiness that I've seen before include "Snugly comfortable quality" , "Warm quality" , "Snug quality" , "Intimacy" , "Comfort, warmth" .)