Empress’s army once carrying nerve gas (7)
I believe the answer is:
tsarina
'empress's' is the definition.
(I've seen this before)
'army once carrying nerve gas' is the wordplay.
'army once' becomes 'ta' (I am not sure about this - if you are sure you should believe this answer much more).
'carrying' is an insertion indicator.
'nerve gas' becomes 'sarin' (I've seen this before).
'ta' going around 'sarin' is 'TSARINA'.
Can you help me to learn more?
(Other definitions for tsarina that I've seen before include "Monarch" , "Catherine, say" , "Foreign ruler" , "Wife of royal Russian" , "aristocratic woman" .)