Most unpleasant sparkling wine in Robin's home? (8)
I believe the answer is:
nastiest
'most unpleasant' is the definition.
('nasty' can be similar in meaning to 'unpleasant')
'sparkling wine in robin's home?' is the wordplay.
'sparkling wine' becomes 'asti' (Asti is a type of sparkling wine).
'in' indicates putting letters inside.
'robin's home?' becomes 'nest' (I've seen this before).
'asti' going inside 'nest' is 'NASTIEST'.
(Other definitions for nastiest that I've seen before include "Most revolting" , "Extremely unpleasant" .)