Big beast thus disposed of? (5)
I believe the answer is:
bison
'big beast' is the definition.
(I've seen this before)
'thus disposed of?' is the wordplay.
I cannot quite understand how this works, but
'thus' could be 'so' ('so' can be a synonym of 'thus') and 'so' is present in the answer.
The remaining letters 'bin' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for bison that I've seen before include "European buffalo" , "Wild cow" , "O bins could be full of buffalo" , "It's a buffalo in America" , "Large shaggy wild ox" .)