The Russian firm’s got the bounce! (7)
I believe the answer is:
cossack
'the russian' is the definition.
(I've seen this before)
'firm's got the bounce' is the wordplay.
'firm's' becomes 'co's' (a firm is a company).
'got the bounce' becomes 'sack' (I can't explain this - if you can you should believe this answer much more).
'cos'+'sack'='COSSACK'
Can you help me to learn more?
(Other definitions for cossack that I've seen before include "soldier" , "Member of an elite cavalry in tzarist Russia" , "Belligerent Russian" , "Member of Russian, Ukrainian people" , "Tsarist cavalryman" .)