Alarm at hearing of poison (6)
I believe the answer is:
tocsin
'alarm' is the definition.
(I know that tocsin is a type of alarm)
'hearing of poison' is the wordplay.
'hearing of' indicates a 'sounds like' (homophone) clue.
'poison' becomes 'toxin' (toxin is a kind of poison).
'toxin' sounds like 'TOCSIN'.
'at' is the link.
(Other definitions for tocsin that I've seen before include "The sound of an alarm bell" , "An alarm bell or signal" , "Con, sit and produce an alarmbell" , "Alarm bell to be found in Scot" , "alarm call" .)