Mixer’s temperature — chilled, mostly (5)
I believe the answer is:
tonic
'mixer's' is the definition.
(I've seen this before)
'temperature chilled mostly' is the wordplay.
'temperature' becomes 't' (abbreviation).
'chilled' becomes 'on ice' (chilled with ice).
'mostly' means to remove the last letter (most but not all of the word).
'onice' with its final letter removed is 'onic'.
't'+'onic'='TONIC'
(Other definitions for tonic that I've seen before include "Quinine water" , "Accompaniment to gin?" , "__water; gin mixer" , "requirement for invigoration?" , "Producing tension of muscles" .)