Chuck eating very big animal (5)
I believe the answer is:
bison
'big animal' is the definition.
'bison' can be an answer for 'animal' (I've seen this before). I am unsure of the 'big' bit.
'chuck eating very' is the wordplay.
'chuck' becomes 'bin' ('bin' can be used as a verb).
'eating' is an insertion indicator.
'very' becomes 'so' (both can mean 'extremely').
'bin' enclosing 'so' is 'BISON'.
Can you help me to learn more?
(Other definitions for bison that I've seen before include "Ox-like ruminant" , "European beast" , "US wild ox" , "Kind of shaggy cattle" , "Shaggy-haired wild ox" .)