They keep a check on a dissipated siren (5)

I believe the answer is:

reins

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'they keep a check' is the definition.
The definition suggests an adverb but the answer is not.

'dissipated siren' is the wordplay.
'dissipated' is an anagram indicator.
'siren' with letters rearranged gives 'REINS'.

'on a' is the link.

Can you help me to learn more?

(Other definitions for reins that I've seen before include "Control for horses" , "Means of checking" , "Rider's straps" , "Harnesses" , "Means of controlling a horse" .)

I've seen this clue in the Evening Standard.

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