Little devils at one mile per second! (4)
I believe the answer is:
imps
'little devils' is the definition.
(I've seen this before)
'at one mile per second' is the wordplay.
'at' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'mile' becomes 'm' (common abbreviation).
'per second' becomes 'ps' (this could be a standard abbreviation I've not previously seen).
'i' next to 'm' is 'im'.
'im'+'ps'='IMPS'
Can you help me to learn more?
(Other definitions for imps that I've seen before include "Demons" , "Cheeky little devils" , "Little mischievous devils" , "Children causing trouble" , "Mischievous spirits" .)