A single beast isn't exactly a herd! (4)
I believe the answer is:
head
'a single' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for head that I've seen before include "See 12" , "Leader; water pressure" , "Promontory; foam" , "River source; promontory" , "Leader; promontory" .)