Cold drink pumped with gas (5)
I believe the answer is:
rheum
'cold' is the definition.
(I've seen this before)
'drink pumped with gas' is the wordplay.
'drink' becomes 'rum' (rum is a kind of drink).
'pumped with' indicates putting letters inside (inserted letters are pumped inside).
'gas' becomes 'He' (chemical symbol for Helium).
'rum' enclosing 'he' is 'RHEUM'.
(Other definitions for rheum that I've seen before include "Catarrh" , "Watery fluid" , "Poetic tears" , "nasal discharge" , "Watery stuff" .)