Cold drink containing gas (5)
I believe the answer is:
rheum
'cold' is the definition.
(I've seen this in another clue)
'drink containing gas' is the wordplay.
'drink' becomes 'rum' (rum is a kind of drink).
'containing' is an insertion indicator.
'gas' becomes 'He' (chemical symbol for helium).
'rum' going around 'he' is 'RHEUM'.
(Other definitions for rheum that I've seen before include "nasal drops" , "cold once upon a time" , "Watery discharge from eyes or nose" , "Watery stuff" , "Catarrh" .)