She has the master key (3)
I believe the answer is:
mae
'she' is the definition.
(I've seen this before)
'the master key' is the wordplay.
'the master' becomes 'MA' (Master of Arts).
'key' becomes 'e' (musical key).
'ma'+'e'='MAE'
'has' acts as a link.
(Other definitions for mae that I've seen before include "- West, US film actress" , "Girl" , "more Scots" , "The first Hollywood blonde femme fatale, . . . West" , "Woman" .)