Note taken from very friendly prisoner (6)
I believe the answer is:
inmate
'prisoner' is the definition.
(inmate can mean someone in prison)
'note taken from very friendly' is the wordplay.
'note' becomes 'ti' (musical note as in do re mi fa so la ti).
'taken from' indicates named letters should be taken away.
'very friendly' becomes 'intimate' (I've seen this before).
'intimate' with 'ti' taken away is 'INMATE'.
(Other definitions for inmate that I've seen before include "Cell resident" , "Con" , "Inhabitant of prison or other institution" , "Inhabitant of prison or hospital" , "Convict" .)