Topping of dark rum on the Parisian's piece of cake (6)
I believe the answer is:
doddle
'piece of cake' is the definition.
(both can mean something very easy)
'topping of dark rum on the parisian's' is the wordplay.
'topping of dark rum' becomes 'dodd' (I am not sure about this - if you are sure you should believe this answer much more).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'the parisian' becomes 'le' (definite article in French).
'dodd'+'le'='DODDLE'
Can you help me to learn more?
(Other definitions for doddle that I've seen before include "Absurdly simple" , "Something easily done" , "Snip" , "Easy crossword?" , "Piece of cake" .)