What’s in large bottle? Some swill it: retsina! (5)
I believe the answer is:
litre
'what's in large bottle?' is the definition.
(litre is a volume of liquid)
'some swill it retsina' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'LITRE' is hidden within 'swill it retsina'.
(Other definitions for litre that I've seen before include "Unit of capacity in metric system" , "Fluid measure" , "Metric measure of capacity" , "Metric liquid measure" , "less than two pints" .)