Four-legged race! (5)
I believe the answer is:
relay
'race' is the definition.
(relay race)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for relay that I've seen before include "Team of workers" , "track event" , "Receive and pass on - race" , "article in bank" , "supplementary store of anything" .)