Boisterous girl’s play about old doctor (6)
I believe the answer is:
tomboy
'boisterous girl's' is the definition.
(term for a boyish girl)
'play about old doctor' is the wordplay.
I cannot quite understand how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found in the answer.
'doctor' could be 'mb' (Bachelor of Medicine, Medicinae Baccalaureus) and 'mb' is present in the answer.
'play' could be 'toy' (synonyms) and 'toy' is found in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
(Other definitions for tomboy that I've seen before include "Wild, boisterous girl" , "Girl who enjoys boyish games" , "Girl said to be exhibiting boyish behaviour" , "Girl in trousers, perhaps?" , "Hoyden" .)