Close friend of prisoner hiding note (8)
I believe the answer is:
intimate
'close friend' is the definition.
(thesaurus)
'prisoner hiding note' is the wordplay.
'prisoner' becomes 'inmate' (inmate is a kind of prisoner).
'hiding' indicates putting letters inside.
'note' becomes 'ti' (I've seen this in other clues**).
'inmate' placed around 'ti' is 'INTIMATE'.
'of' is the link.
(Other definitions for intimate that I've seen before include "Hint to a familiar friend" , "Friendly" , "Suggest to trusted friend?" , "Close acquaintance" , "Give hint to close friend?" .)