Friendly note one included in telegram (8)
I believe the answer is:
sociable
'friendly' is the definition.
('sociable' can be a synonym of 'friendly')
'note one included in telegram' is the wordplay.
'note' becomes 'so' (musical note**).
'one' becomes 'i' (Roman numeral).
'included in' means one lot of letters goes inside another.
'telegram' becomes 'cable' (cable is a kind of telegram).
'i' put into 'cable' is 'ciable'.
'so'+'ciable'='SOCIABLE'
(Other definitions for sociable that I've seen before include "Friendly and pleasant" , "Easy in company" , "Fond of others' company" , "Companionable, friendly" , "Fond of mixing with others" .)