Finished short musical composition, certainly not before time (7)
I believe the answer is:
overdue
'certainly not before time' is the definition.
'finished short musical composition' is the wordplay.
'finished' becomes 'over' (over can mean finished or ended).
'short' means to remove the last letter.
'musical composition' becomes 'duet' (duet is a kind of musical composition).
'duet' with its last letter taken off is 'due'.
'over'+'due'='OVERDUE'
(Other definitions for overdue that I've seen before include "Past the ETA" , "Late for payment" , "Past the appointed date for payment" , "Beyond the expected time" , "Behindhand" .)