Like sports drink, one drunk on ice, briefly (8)
I believe the answer is:
Here is my best explanation:
'like sports drink' is the definition.
'one drunk on ice briefly' is the subsidiary indication.
'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 last letter removed is 'ic'.
(Other definitions for isotonic that I've seen before include "equally tense", "Powerade, say", "Having the same tension", "as energy drinks might be", "(Of muscle action) taking place with normal contraction", "Type of drink restoring body fluids and salts", "drink sold at the leisure centre", "sort of muscle action", "at the same pressure?" and "(Of a drink) having the same concentrations of salts and minerals as in the body".)