Old fellow goes after support for big beast (8)
I believe the answer is:
mastodon
'big beast' is the definition.
(I've seen this before)
'old fellow goes after support' is the wordplay.
'old' becomes 'o' (abbreviation).
'fellow' becomes 'don' (both can mean an academic).
'goes after' says to put letters next to each other.
'support' becomes 'mast' ().
'o'+'don'='odon'
'odon' put after 'mast' is 'MASTODON'.
'for' acts as a link.
(Other definitions for mastodon that I've seen before include "very old elephant" , "Prehistoric creature" , "Large mammal" , "Extinct mammal like an elephant" , "Extinct elephant ancestor" .)