Coach under way past square (4-2-4)

I believe the answer is:

four-in-hand

'coach' is the definition.
(synonyms)

'under way past square' is the wordplay.
'under way' becomes 'in hand' (both can mean in progress).
'past' says to put letters next to each other.
'square' becomes 'four' (four is a square number).
'inhand' put after 'four' is 'FOUR-IN-HAND'.

I've seen this clue in The Times.

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