It’s deadly earnest, in essence, to put dress on (5)
I believe the answer is:
sarin
'it's deadly' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'earnest in essence to put dress on' is the wordplay.
'in essence' means to look at the middle letters.
'to put dress' becomes 'sari' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' is a charade indicator (letters next to each other).
The central letter of 'earnest' is 'n'.
'n' put after 'sari' is 'SARIN'.
Can you help me to learn more?
(Other definitions for sarin that I've seen before include "Lethal nerve-gas" , "Gas used in Tokyo subway attack in 1995" , "Nerve gas - rains (anag)" .)