Restriction of note in written material (5)
I believe the answer is:
limit
'restriction' is the definition.
(I've seen this in another clue)
'note in written material' is the wordplay.
'note' becomes 'mi' (musical note).
'in' indicates putting letters inside.
'written material' becomes 'lit' (short for literature).
'mi' inserted inside 'lit' is 'LIMIT'.
'of' is the link.
(Other definitions for limit that I've seen before include "As far as something can go" , "Curb" , "Boundary, demarcation line" , "Restrict - boundary" , "Put restriction on" .)