Doctor takes in large old musical (7)
I believe the answer is:
melodic
'musical' is the definition.
(I know that musical can be written as melodic)
'doctor takes in large old' is the wordplay.
'doctor' becomes 'medic' (doctor is a type of medic).
'takes in' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'l'+'o'='lo'
'medic' enclosing 'lo' is 'MELODIC'.
(Other definitions for melodic that I've seen before include "Pleasant-sound" , "Old mice (anag.)" , "Harmonious" , "Of music, tuneful and sweet" , "Pleasant to listen to" .)