Friend since penning note (5)
I believe the answer is:
amigo
'friend' is the definition.
('friend' in Spanish)
'since penning note' is the wordplay.
'since' becomes 'ago' (I've seen this before).
'penning' means one lot of letters goes inside another.
'note' becomes 'mi' (as in do-re-mi).
'ago' placed around 'mi' is 'AMIGO'.
(Other definitions for amigo that I've seen before include "Friend in Spanish-speaking areas" , "Foreign ally" , "friend from Barcelona" , "Friend, comrade" , "friend in Uruguay" .)