Old court letter's content and composition (7)
I believe the answer is:
octette
'composition' is the definition.
(I know that octette is a type of composition)
'old court letter's content' is the wordplay.
'old' becomes 'o' (abbreviation).
'court' becomes 'ct' (abbreviation used in road names).
'content' indicates the central letters.
The middle letters of 'letter' are 'ette'.
'o'+'ct'+'ette'='OCTETTE'
'and' acts as a link.
(Other definitions for octette that I've seen before include "Noted work" , "Eight musicians" , "Group of eight" .)