Crossword Genius

Expert teacher now retired (4,6)

Ross

I think it could be either:

Which length description are you looking for?:

10

4,6

past master

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'expert' is the definition.
(I know that past master is a type of expert)

'teacher now retired' is the wordplay.
I cannot quite understand how this works, but
'teacher' could be 'master' (I've seen this before) and 'master' is found in the answer.
The remaining letters 'past' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for past master that I've seen before include "Thomas Arnold was" , "seen at staff reunions?" , "Adept fellow" , "Person experienced in a particular field" , "wizard" .)

I've seen this clue in The Globe and Mail.
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