Little devils at one mile per second! (4)

I believe the answer is:

imps

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'little devils' is the definition.
(I've seen this in another clue)

'at one mile per second' is the wordplay.
'at' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'mile' becomes 'm' (common abbreviation).
'per second' becomes 'ps' (this might be a standard abbreviation of which I'm unaware).
'i' put next to 'm' is 'im'.
'im'+'ps'='IMPS'

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(Other definitions for imps that I've seen before include "Demons" , "Cheeky little devils" , "Little mischievous devils" , "Children causing trouble" , "Mischievous spirits" .)

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