Ruins trifle -- odd ingredients should have been discarded before (5)
I believe the answer is:
until
'before' is the definition.
(both can mean up to a certain point)
'ruins trifle odd ingredients should have been discarded' is the wordplay.
'odd ingredients should have been discarded' indicates alternate letters (the letters in odd-numbered positions are removed).
'ruins'+'trifle'='ruinstrifle'
The alternate letters of 'ruinstrifle' are 'UNTIL'.
(Other definitions for until that I've seen before include "Not beyond (hour)" , "Pending such time as" , "Unlit (anag.)" , "Up to that moment" , "Up to (a point in time)" .)