Scot holding the master key? (3)
I believe the answer is:
mac
'scot' is the definition.
(many Scottish surnames begin with Mac)
'the master key?' is the wordplay.
'the master' becomes 'MA' (Master of Arts).
'key?' becomes 'c' (musical key).
'ma'+'c'='MAC'
'holding' is the link.
(Other definitions for mac that I've seen before include "Raincoat (colloq.)" , "PC" , "Overcoat" , "Coat - computer" , "Waterproof coat (colloq.)" .)