Drink can go cold when served up (6)
I believe the answer is:
cognac
'drink' is the definition.
(Cognac is an example)
'can go cold when served up' is the wordplay.
'cold' becomes 'C' (eg on taps).
'when served up' says the letters should be written in reverse.
'can'+'go'+'c'='cangoc'
'cangoc' reversed gives 'COGNAC'.
(Other definitions for cognac that I've seen before include "strong drink" , "High-quality brandy" , "Good brandy" , "Brandy as for Napoleon (6)." , "High quality brandy" .)