Master Charlie's coat (3)
I believe the answer is:
mac
'coat' is the definition.
(mac is a kind of coat)
'master charlie's' is the wordplay.
'master' becomes 'ma'.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'ma'+'c'='MAC'
(Other definitions for mac that I've seen before include "PC" , "It may help you keep dry" , "laptop?" , "Waterproof raincoat, in short" , "Coat - computer" .)