Poison? Sound the alarm (5)
'poison?' is the definition.
(I know that toxin is a type of poison)
'sound the alarm' is the wordplay.
'sound' shows a homophone (sound like).
'the alarm' becomes 'tocsin' (tocsin is a kind of alarm).
'tocsin' sounds like 'TOXIN'.
(Other definitions for toxin that I've seen before include "Form of poison" , "Poison of organic origin" , "Poisonous substance" , "Natural poison" , "Poison, venom" .)
'alarm' is the definition.
(I know that tocsin is a type of alarm)
'poison sound' is the wordplay.
'poison' becomes 'toxin' (toxin is a kind of poison).
'sound' indicates a 'sounds like' (homophone) clue.
'toxin' is a homophone of 'TOCSIN'.
'the' is the link.
(Other definitions for tocsin that I've seen before include "Alarm bell to be found in Scot" , "An alarm bell or signal" , "Warning bell" , "Con, sit and produce an alarmbell" , "alarm call" .)