Toxin mostly abundant at home (5)
I believe the answer is:
ricin
'toxin' is the definition.
(I know that ricin is a type of toxin)
'mostly abundant at home' is the wordplay.
'mostly' means to remove the last letter.
'abundant' becomes 'rich' (associated in meaning).
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'rich' with its final letter taken away is 'ric'.
'ric'+'in'='RICIN'
(Other definitions for ricin that I've seen before include "Poison from castor oil seeds" , "Toxic stuff" , "Dangerous poison" , "toxic substance" , "castor-oil product" .)