Prisoner possibly penning note for close friend (8)
I believe the answer is:
intimate
'close friend' is the definition.
(intimate can be a noun: a close friend)
'prisoner possibly penning note' is the wordplay.
'prisoner possibly' becomes 'inmate' (I can't explain this - if you can you should believe this answer much more).
'penning' means one lot of letters goes inside another.
'note' becomes 'ti' (musical note, as in do re mi so fa la ti do).
'inmate' placed around 'ti' is 'INTIMATE'.
'for' is the link.
Can you help me to learn more?
(Other definitions for intimate that I've seen before include "Make known" , "Suggest to trusted friend?" , "pal" , "cosy" , "Close acquaintance" .)