Checks a broken siren (5)
I believe the answer is:
reins
'checks' is the definition.
(reining is a kind of checking)
'a broken siren' is the wordplay.
'a broken' indicates an anagram.
'siren' anagrammed gives 'REINS'.
(Other definitions for reins that I've seen before include "Rider's straps" , "Bridle straps" , "Control for horses" , "Means of checking" , "Checks" .)