Leading horologist is early (5,2,4)

I believe the answer is:

ahead of time

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

'leading horologist' is the wordplay.
'leading' becomes 'ahead of' (I've seen this in another clue).
'horologist' becomes 'time' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'aheadof'+'time'='AHEAD OF TIME'

Can you help me to learn more?

(Another definition for ahead of time that I've seen is " Early".)

I've seen this clue in The Daily Express.

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