Mum's fuss with knight and old beast (8)
I believe the answer is:
mastodon
'old beast' is the definition.
'mastodon' can be an answer for 'beast' (mastodon is a kind of beast). I'm not sure about the 'old' bit.
'mum's fuss with knight' is the wordplay.
'mum's' becomes 'ma's' (ma can mean mother).
'fuss' becomes 'todo' (both can mean a commotion).
'with' is a charade indicator (letters next to each other).
'knight' becomes 'N' (chess abbreviation).
'mas'+'todo'+'n'='MASTODON'
'and' is the link.
Can you help me to learn more?
(Other definitions for mastodon that I've seen before include "Man stood like extinct sort of elephant" , "Extinct elephant-like mammal" , "Extinct mammal like an elephant" , "Large extinct animal resembling elephant" , "old jumbo" .)