Officers riding free within two seconds (8)

I believe the answer is:

mounties

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'officers' is the definition.
(Mountie is an example)

'riding free within two seconds' is the wordplay.
'two seconds' can mean two substitutions for 'second'.
'riding free' becomes 'untie' (I've seen this before. I am not sure about the 'riding' bit.).
'within' means one lot of letters goes inside another.
'second' becomes 'mo' (both are short periods of time).
'second' becomes 's' (common abbreviation).
'mo'+'s'='mos'
'untie' put into 'mos' is 'MOUNTIES'.

Can you help me to learn more?

(Other definitions for mounties that I've seen before include "Canadian federal police" , "Police force of Canada" .)

I've seen this clue in The Telegraph.

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