History teacher, a wizard (4,6)
I believe the answer is:
past master
'a wizard' is the definition.
(thesaurus)
'history teacher' is the wordplay.
'history' becomes 'past' (I've seen this before).
'teacher' becomes 'master' (I've seen this before).
'past'+'master'='PAST-MASTER'
(Other definitions for past master that I've seen before include "Person experienced in a particular field" , "Expert - former office holder" , "seen at staff reunions?" , "Someone with a long and thorough experience in a given activity" , "wizard" .)