Riddle of sister tucking into fish and strong drink (9)
I believe the answer is:
conundrum
'riddle' is the definition.
('conundrum' can be a synonym of 'riddle')
'sister tucking into fish and strong drink' is the wordplay.
'sister' becomes 'nun' (I've seen this before).
'tucking into' means one lot of letters goes inside another.
'fish' becomes 'cod' (cod is a kind of fish).
'and' says to put letters next to each other.
'strong drink' becomes 'rum' (rum is a kind of strong drink).
'nun' going inside 'cod' is 'conund'.
'conund'+'rum'='CONUNDRUM'
'of' acts as a link.
(Other definitions for conundrum that I've seen before include "It's a riddle or a hard question" , "Puzzling question" , "That's puzzling" , "Puzzle, riddle" , "Difficult problem" .)