One drunk on one cold type of drink (8)
I believe the answer is:
isotonic
'type of drink' is the definition.
(I've seen this before)
'one drunk on one cold' is the wordplay.
'one' becomes 'i' (Roman numeral).
'drunk' becomes 'sot' (sot can mean a drunken person).
'one' becomes 'i' (Roman numeral).
'cold' becomes 'C' (eg on taps).
'i'+'sot'+'on'+'i'+'c'='ISOTONIC'
(Other definitions for isotonic that I've seen before include "Powerade, say" , "sort of muscle action" , "Type of drink restoring body fluids and salts" , "(Of muscle action) taking place with normal contraction" , "drink sold at the leisure centre" .)