Sweetener when wise man loses head over rotten meat (9)
I believe the answer is:
Here is my best explanation:
'sweetener' is the definition.
(aspartame is a kind of sweetener)
'wise man loses head over rotten meat' is the subsidiary indication.
'wise man' becomes 'caspar' (I've seen this before).
'loses head' means to remove the first letter.
'over' says to put letters next to each other.
'rotten' is an anagram indicator.
'caspar' with its first letter taken off is 'aspar'.
'meat' with letters rearranged gives 'tame'.
'when' is the link.
(Other definitions for aspartame that I've seen before include "Strong artificial sweetener -- same apart (anag)", "What sweetens", "Artificial sweetener" and "Sugar substitute".)