Russian bird holds fool back (7)
I believe the answer is:
cossack
'russian' is the definition.
(I've seen this before)
'bird holds fool back' is the wordplay.
'bird' becomes 'cock' (cock is a kind of bird).
'holds' is an insertion indicator.
'fool' becomes 'ass' (both can mean a stupid person).
'back' shows that the letters should be reversed in order.
'ass' written backwards gives 'ssa'.
'cock' going around 'ssa' is 'COSSACK'.
(Other definitions for cossack that I've seen before include "One from southern Russia or Ukraine noted for horse riding skills" , "Tsarist cavalryman" , "soldier" , "Member of an elite cavalry in tzarist Russia" , "Warrior peasant of SE Russia" .)