One really invigorating kind of drink (8)
I believe the answer is:
'kind of drink' is the definition.
(drink drunk after exercise)
'one really invigorating' is the wordplay.
'one' becomes 'i' (Roman numeral).
'really' becomes 'so' (both can mean 'extremely').
'invigorating' becomes 'tonic' (similar in meaning).
(Other definitions for isotonic that I've seen before include "Powerade, say" , "Having the same tension" , "Type of drink restoring body fluids and salts" , "sort of muscle action" , "at the same pressure?" .)