Crossword Genius

Not up for one cold drink (5)

Ross

I believe the answer is:

tonic

'drink' is the definition.
(tonic is a kind of drink)

'not up for one cold' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'for' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'cold' becomes 'C' (eg on taps).
'not' in reverse letter order is 'ton'.
'ton'+'i'+'c'='TONIC'

(Other definitions for tonic that I've seen before include "__water; gin mixer" , "Restorative that invigorates" , "Small drink of spirits" , "Carbonated mixer" , "Quinine water" .)

I've seen this clue in The Mirror.
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