Brazilian city surrounded by marvellous lake (8)
I believe the answer is:
superior
'lake' is the definition.
(I've seen this before)
'brazilian city surrounded by marvellous' is the wordplay.
'brazilian city' becomes 'rio' (I've seen this before).
'surrounded by' indicates putting letters inside.
'marvellous' becomes 'super' (I've seen this before).
'rio' placed inside 'super' is 'SUPERIOR'.
(Other definitions for superior that I've seen before include "Snooty" , "head of a religious order" , "Mother is sometimes" , "Better than, in a higher class" , "World's largest freshwater lake" .)