Animal to risk swallowing arsenic (5)
I believe the answer is:
beast
'animal' is the definition.
('beast' can be a synonym of 'animal')
'risk swallowing arsenic' is the wordplay.
'risk' becomes 'bet'.
'swallowing' means one lot of letters goes inside another.
'arsenic' becomes 'as' (As is the chemical symbol for arsenic).
'bet' placed around 'as' is 'BEAST'.
'to' acts as a link.
(Other definitions for beast that I've seen before include "Brute; creature" , "Large animal -- objectionable person" , "Monstrous person" , "horse?" , "rotter" .)