A classy type of Don is behind one of those containing 3 down, for instance (12)

I believe the answer is:

schoolfellow

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'one of those containing 3 down for instance' is the definition.
I can't tell whether this defines the answer.

'a classy type of don is behind' is the wordplay.
'a classy type of' becomes 'school' (I can't explain this - if you can you should give a lot more credence to this answer).
'don' becomes 'fellow' (I've seen this before).
'is behind' is a charade indicator (letters next to each other).
'school'+'fellow'='SCHOOLFELLOW'

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I've seen this clue in the Irish Times.

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