Pianist, old, good teacher (5)

I believe the answer is:

ogdon

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

'old good teacher' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'good' becomes 'g' (abbreviation).
'teacher' becomes 'don' (don is a kind of teacher).
'o'+'g'+'don'='OGDON'

(Another definition for ogdon that I've seen is " Pianist".)

I've seen this clue in The Telegraph.

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