Favourite role, not finishing off juice (6)
I believe the answer is:
petrol
'juice' is the definition.
(juice can informally mean petrol)
'favourite role not finishing off' is the wordplay.
'favourite' becomes 'pet' (synonyms).
'not finishing off' means to remove the last letter.
'role' with its final letter removed is 'rol'.
'pet'+'rol'='PETROL'
(Other definitions for petrol that I've seen before include "Gasoline" , "The juice" , "Mineral oil refined as fuel" , "Fuel for road vehicles" , "Common motor fuel" .)