Cold dessert (6)
'cold dessert' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for trifle that I've seen before include "Dessert; act frivolously" , "Act foolishly" , "Small amount; dish" , "Type of dessert" , "Pudding - bagatelle" .)
(Other definitions for blancmange that I've seen before include "Dessert of flavoured milk stiffened with cornflower" , "Jelly-like pudding made with milk" , "Sweet dessert" , "Jelly-like dessert" , "Wobbly dessert" .)