Don’t object so much? That’s stupid (8)
I believe the answer is:
mindless
'stupid' is the definition.
(I've seen this before)
'don't object so much?' is the wordplay.
I cannot quite understand how this works, but
'object' could be 'nd' (Nd is an example) and 'nd' is present in the answer.
This explanation may well be incorrect...
'that's' is the link.
(Other definitions for mindless that I've seen before include "without reason" , "Without thought or care" , "Lacking intelligence, asinine" , "Stupid" , "Irrational" .)