As behind, starts on numerous Indy crosswords (7)
I believe the answer is:
arsenic
'as' is the definition.
('arsenic' can be a synonym of 'as')
'behind starts on numerous indy crosswords' is the wordplay.
'behind' becomes 'arse' ('arse' can be a synonym of 'behind').
'starts on' indicates taking the first letters.
The first letters of 'numerous indy crosswords' is 'nic'.
'arse'+'nic'='ARSENIC'
(Other definitions for arsenic that I've seen before include "Powerful poison" , "Toxic metallic element used as rat poison" , "killer" , "Poisons in scare" , "Poison used in pesticides and weedkillers" .)