Crossword Genius

Expert teacher, now retired (10)

Ross

I think it could be either:

Which length description are you looking for?:

10

4,6

pastmaster

I'm a little stuck... Click here to teach me more about this clue!

'expert' is the definition.
(I've seen this before)

'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 present 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 pastmaster that I've seen before include "Expert" , "Old hand" .)

I've seen this clue in The Irish Independent.
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