Beast is fox stoneworker catches (8)
I believe the answer is:
mastodon
'beast' is the definition.
(mastodon is a kind of beast)
'fox stoneworker catches' is the wordplay.
'fox' becomes 'tod' (historic term).
'stoneworker' becomes 'mason' (I've seen this before).
'catches' is an insertion indicator.
'tod' going within 'mason' is 'MASTODON'.
'is' acts as a link.
(Other definitions for mastodon that I've seen before include "Extinct mammal - man stood (anag)" , "Man stood like extinct sort of elephant" , "Large extinct animal resembling elephant" , "Big beast" , "Prehistoric creature" .)