Novel medieval instrument starts to come apart (7)
I believe the answer is:
rebecca
'novel' is the definition.
(Daphne du Maurier novel)
'medieval instrument starts to come apart' is the wordplay.
'medieval instrument' becomes 'rebec' (I've seen this in another clue).
'starts to' suggests taking the first letters.
The initial letters of 'come apart' is 'ca'.
'rebec'+'ca'='REBECCA'
(Other definitions for rebecca that I've seen before include "also Isaac" , "Du Maurier novel - Hitchcock thriller" , "Daphne du Maurier book; girl's name" , "Daphne du Maurier novel; girl's name" , "Maxim she betrayed" .)