Riddle of sister surrounded by fish and alcoholic drink (9)
I believe the answer is:
conundrum
'riddle' is the definition.
('conundrum' can be a synonym of 'riddle')
'sister surrounded by fish and alcoholic drink' is the wordplay.
'sister' becomes 'nun' (I've seen this before).
'surrounded by' is an insertion indicator.
'fish' becomes 'cod' (cod is a kind of fish).
'and' is a charade indicator (letters next to each other).
'alcoholic drink' becomes 'rum' (I've seen this before**).
'nun' inserted within 'cod' is 'conund'.
'conund'+'rum'='CONUNDRUM'
'of' acts as a link.
(Other definitions for conundrum that I've seen before include "Puzzle, riddle" , "That's puzzling" , "It's a riddle or a hard question" , "Puzzling problem" , "Poser" .)