Empress keeps lethal gas in at back (7)
I believe the answer is:
tsarina
'empress' is the definition.
(I know that tsarina is a type of empress)
'lethal gas in at back' is the wordplay.
'lethal gas' becomes 'sarin' (I've seen this in another clue).
'in' indicates putting letters inside.
'back' says the letters should be written backwards.
'at' back-to-front is 'ta'.
'sarin' inserted inside 'ta' is 'TSARINA'.
'keeps' acts as a link.
(Other definitions for tsarina that I've seen before include "Russian empress (S, not Z)" , "Wife of an old Russian emperor" , "Wife of a tsar" , "Monarch" , "companion of emperor" .)