Close friend slips note into prisoner (8)
I believe the answer is:
intimate
'close friend' is the definition.
(intimate can be a noun: a close friend)
'note into prisoner' is the wordplay.
'note' becomes 'ti' (musical note as in do re mi fa so la ti).
'into' indicates putting letters inside.
'prisoner' becomes 'inmate' (inmate can mean someone in prison).
'ti' put into 'inmate' is 'INTIMATE'.
'slips' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for intimate that I've seen before include "pal" , "Personal - hint - friend" , "Hint to a familiar friend" , "cosy" , "Tip off" .)