One on hand for a major road race? (4)

I believe the answer is:

mitt

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'one on hand' is the definition.
'mitt' can be an answer for 'hand' (synonyms). I'm unsure of the 'one on' bit.

'a major road race?' is the wordplay.
'a major road' becomes 'MI' (resembles M1, English motorway).
'race?' becomes 'tt'.
'mi'+'tt'='MITT'

'for' is the link.

Can you help me to learn more?

(Another definition for mitt that I've seen is " Baseball glove".)

I've seen this clue in the Evening Standard.

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