New dress before becoming lethal agent (5)
I believe the answer is:
sarin
'lethal agent' is the definition.
I know nothing about this answer so I can't tell whether this works.
'new dress before' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'dress' becomes 'sari' (sari is a kind of dress).
'before' means one lot of letters go next to another.
'n' put after 'sari' is 'SARIN'.
'becoming' acts as a link.
Can you help me to learn more?
(Other definitions for sarin that I've seen before include "Nerve gas - rains (anag)" , "Lethal nerve-gas" , "Gas used in Tokyo subway attack in 1995" .)