Jack, posh, takes something cold in drink (5)
I believe the answer is:
juice
'drink' is the definition.
(I've seen this before)
'jack posh takes something cold' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'takes' is a charade indicator (letters next to each other).
'something cold' becomes 'ice'.
'j'+'u'+'ice'='JUICE'
'in' acts as a link.
(Other definitions for juice that I've seen before include "Fruity liquid" , "It can be expressed" , "Vitality" , "Energy" , "Sap; petrol" .)