The master, a college head, is a Scot (3)
I believe the answer is:
mac
'scot' is the definition.
(many Scottish surnames begin with Mac)
'the master a college head' is the wordplay.
'the master' becomes 'MA' (Master of Arts).
'a college head' becomes 'c' (I've seen this in other clues).
'ma'+'c'='MAC'
'is a' is the link.
(Other definitions for mac that I've seen before include "Coat - computer" , "A rainproof" , "PC" , "Waterproof raincoat, in short" , "Waterproof coat (colloq.)" .)