Sickeningly ordered a sundae with a side of cream (2,7)
I believe the answer is:
ad nauseam
'sickeningly' is the definition.
(I've seen this in another clue)
'ordered a sundae with a side of cream' is the wordplay.
'ordered' indicates an anagram.
'with' is a charade indicator (letters next to each other).
'side of' says to take the final letters.
The last letter of 'cream' is 'm'.
'a'+'sundae'='asundae'
'asundae' with letters rearranged gives 'adnause'.
'adnause'+'a'+'m'='AD NAUSEAM'
(Other definitions for ad nauseam that I've seen before include "To a tiresomely excessive degree" , "Much too much" , "To the point of disgust" , "Done so often that it becomes tiresome" , "To a disgusting extent" .)