A length in preparation (5)

I believe the answer is:

afoot

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

'a length' is the wordplay.
'length' becomes 'foot' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'foot'='AFOOT'

Can you help me to learn more?

(Other definitions for afoot that I've seen before include "Progressing, in operation - like Sherlock Holmes's game" , "In the wind" , "Under way (like hunted game?)" , "Stirring" , "Brewing" .)

I've seen this clue in The Irish News.

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