Prattling monarch follows shaggy beast (7)
I believe the answer is:
yakking
'prattling' is the definition.
(both can mean talking excessively)
'monarch follows shaggy beast' is the wordplay.
'monarch' becomes 'king' (king is a kind of monarch).
'follows' is a charade indicator (letters next to each other).
'shaggy beast' becomes 'yak' (type of hairy ox).
'king' put after 'yak' is 'YAKKING'.
(Other definitions for yakking that I've seen before include "Going on" , "having long jaw" , "delivering speech" , "Talking at great length" , "chewing fat" .)