Adept former schoolteacher (4,6)
I believe the answer is:
past master
'adept' is the definition.
(I've seen this before)
'former schoolteacher' is the wordplay.
'former' becomes 'past' (similar in meaning).
'schoolteacher' becomes 'master' (I've seen this before).
'past'+'master'='PAST-MASTER'
(Other definitions for past master that I've seen before include "wizard" , "Adept fellow" , "Person experienced in a particular field" , "Thomas Arnold was" , "Someone with a long and thorough experience in a given activity" .)