Restraint could be prelude to mating (5)
I believe the answer is:
check
'restraint' is the definition.
(check is a kind of restraint)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for check that I've seen before include "threat to king" , "Suppress" , "Verify, examine" , "Investigate; stop" , "Curb" .)