River cruise liner partially capsized (4)
I believe the answer is:
nile
'river' is the definition.
(I know that Nile is a river)
'cruise liner partially capsized' is the wordplay.
'partially' indicates a hidden word.
'capsized' shows that the letters should be reversed in order (in a down clue, the word turns upside down).
'elin' is hidden within 'cruise liner'.
'elin' backwards is 'NILE'.
(Other definitions for nile that I've seen before include "'The longest river, blue and white (4)'" , "Egypt's river" , "River associated with Cleopatra" , "Long river" , "Cleopatra's river" .)