Slavonic firms on fire (7)
I believe the answer is:
cossack
'slavonic' is the definition.
(Cossack is an example)
'firms on fire' is the wordplay.
'firms on' becomes 'cos' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fire' becomes 'sack' (dismiss someone from their job).
'cos'+'sack'='COSSACK'
(Other definitions for cossack that I've seen before include "Russian or Ukrainian cavalryman" , "Belligerent Russian" , "One from southern Russia or Ukraine noted for horse riding skills" , "Russian tribesman famed for horsemanship" , "soldier" .)