Dog worrying young herd (9)
I believe the answer is:
greyhound
'dog' is the definition.
(greyhound is a kind of dog)
'worrying young herd' is the wordplay.
'worrying' indicates an anagram.
'young'+'herd'='youngherd'
'youngherd' anagrammed gives 'GREYHOUND'.
(Other definitions for greyhound that I've seen before include "racer" , "Want to bet on it?" , "Dog trained for racing on tracks" , "Fast-moving dog" , "Tall, slender canine" .)