As a siren going off, head for cover (7)
I believe the answer is:
arsenic
'as' is the definition.
(As is the chemical symbol for arsenic)
'a siren going off head for cover' is the wordplay.
'going off' is an anagram indicator.
'head for' suggests taking the first letters.
The first letter of 'cover' is 'c'.
'siren' is an anagram of 'rseni'.
'a'+'rseni'+'c'='ARSENIC'
(Other definitions for arsenic that I've seen before include "Toxic metalloid element" , "Brittle grey-green semi-metal, atomic number 33" , "Famous stage comedy ''. . . . . . . and Old Lace''" , "Poisons in scare" , "Can rise result from poison?" .)