Spy master is years in disguise (8)
I believe the answer is:
emissary
'spy' is the definition.
(emissary can mean a spy in another country)
'master is years in disguise' is the wordplay.
'master' becomes 'm'.
'in disguise' indicates anagramming the letters.
'm'+'is'+'years'='misyears'
'misyears' with letters rearranged gives 'EMISSARY'.
(Other definitions for emissary that I've seen before include "Government messenger" , "Diplomatic agent" , "Someone sent on a mission" , "Messenger, envoy" , "one dispatched officially" .)