Master Charlie's raincoat (3)
I believe the answer is:
mac
'raincoat' is the definition.
(macintosh is a type of raincoat)
'master charlie's' is the wordplay.
'master' becomes 'ma' (Master of Arts).
'charlie' becomes 'C' (phonetic alphabet: alpha, bravo, charlie etc.).
'ma'+'c'='MAC'
(Other definitions for mac that I've seen before include "A Gaelic son, often in name" , "one used in shower?" , "Waterproof coat (colloq.)" , "Waterproof raincoat, in short" , "laptop?" .)