Empress's army penetrated by nerve gas (7)
I believe the answer is:
tsarina
'empress's' is the definition.
(I've seen this before)
'army penetrated by nerve gas' is the wordplay.
'army' becomes 'TA' (abbreviation for Territorial Army).
'penetrated by' indicates putting letters inside (I've seen this in other clues).
'nerve gas' becomes 'sarin' (I've seen this before).
'ta' enclosing 'sarin' is 'TSARINA'.
(Other definitions for tsarina that I've seen before include "Foreign ruler" , "Catherine, say" , "companion of emperor" , "Russian royal; Sinatra (anag.)" , "Consort" .)