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 wordplay.
'wise man' becomes 'gaspar' (I've seen this before).
'loses head' means to remove the first letter.
'over rotten meat' becomes 'tame' (I can't justify this - if you can you should give a lot more credence to this answer).
'gaspar' with its first letter taken away is 'aspar'.
'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".)