The first to mix? Not us (4)
I believe the answer is:
them
'not us' is the definition.
(I've seen this before)
'the first to mix?' is the wordplay.
'first to' suggests taking the first letters.
The initial letter of 'mix' is 'm'.
'the'+'m'='THEM'
(Other definitions for them that I've seen before include "Certainly not us, the other lot" , "Those people (as object)" , "people objectively" , "... in another case, they" , "Others" .)