Girl swallowing doctor’s instrument (7)
I believe the answer is:
marimba
'instrument' is the definition.
(I know that marimba is a type of percussion instrument)
'girl swallowing doctor's' is the wordplay.
'girl' becomes 'maria' ().
'swallowing' means one lot of letters goes inside another.
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'maria' enclosing 'mb' is 'MARIMBA'.
(Other definitions for marimba that I've seen before include "Latin American instrument like a xylophone" , "Xylophone-like instrument from Africa" , "Jazz instrument" , "Deep toned percussion instrument" , "African xylophone" .)