Washout when siren goes off? (5)
I believe the answer is:
rinse
'washout' is the definition.
(thesaurus)
'siren goes off?' is the wordplay.
'goes off?' indicates anagramming the letters (I've seen 'go off' mean this).
'siren' with letters rearranged gives 'RINSE'.
'when' is the link.
(Other definitions for rinse that I've seen before include "Hair preparation" , "Use water to remove soap" , "Hair colourant" , "Flush" , "Sluice" .)