Got up when siren went off (5)
I believe the answer is:
risen
'got up' is the definition.
('rise' can be a synonym of 'get up')
'siren went off' is the wordplay.
'went off' is an anagram indicator.
'siren' with letters rearranged gives 'RISEN'.
'when' is the link.
(Other definitions for risen that I've seen before include "Increased" , "Moved up" , "stirred" , "Resin (anag.)" , "Above the horizon" .)