Where criminal's sent, not ultimately a mixer (5)
I believe the answer is:
tonic
'mixer' is the definition.
(I've seen this before)
'where criminal's sent not ultimately a' is the wordplay.
I cannot quite see how this works, but
an anagram of 'not' is 'ton' which is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tonic that I've seen before include "Key" , "do for musicians" , "Gin mixer - pick-me-up" , "Invigorating liquid" , "medicinal preparation" .)