Like sports drink, one drunk on ice, briefly (8)
I believe the answer is:
isotonic
'like sports drink' is the definition.
'one drunk on ice briefly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'drunk' becomes 'sot' ('sot' can be a synonym of 'drunk').
'briefly' means to remove the last letter.
'ice' with its final letter taken away is 'ic'.
'i'+'sot'+'on'+'ic'='ISOTONIC'
(Other definitions for isotonic that I've seen before include "equally tense" , "at the same pressure?" , "Powerade, say" , "as energy drinks might be" , "Type of drink restoring body fluids and salts" .)