Expert history teacher? (4,6)
I believe the answer is:
past master
'expert' is the definition.
(I know that past master is a type of expert)
'history teacher?' is the wordplay.
'history' becomes 'past' (I've seen this in another clue).
'teacher?' becomes 'master' (I've seen this before).
'past'+'master'='PAST-MASTER'
(Other definitions for past master that I've seen before include "wizard" , "Thomas Arnold was" , "seen at staff reunions?" , "Someone with a long and thorough experience in a given activity" , "Person experienced in a particular field" .)