Restrain obstinate beast biting a couple of Zulus (6)
I believe the answer is:
muzzle
'restrain' is the definition.
(I know that muzzle is a more specific form of check)
'obstinate beast biting a couple of zulus' is the wordplay.
'a couple of zulus' means two replacements for 'zulu'.
'obstinate beast' becomes 'mule' (I have seen 'Stubborn beast' mean 'mule' so perhaps 'beast' could also mean 'mule'. I am not sure about the 'obstinate' bit.).
'biting' means one lot of letters goes inside another (some letters bite or swallow others).
'zulu' becomes 'Z' (phonetic alphabet: alpha, bravo, charlie etc.).
'zulu' becomes 'Z' (phonetic alphabet: alpha, bravo, charlie etc.).
'z'+'z'='zz'
'mule' enclosing 'zz' is 'MUZZLE'.
Can you help me to learn more?
(Other definitions for muzzle that I've seen before include "Dog's jaws and nose" , "Prevent from speaking out" , "Snout - front end of a gun barrel" , "Dog's jaw; to silence" , "Open end of a gun barrel" .)