Like some American lowlife, finally offering a sweetener (9)
I believe the answer is:
aspartame
'a sweetener' is the definition.
(aspartame is a kind of sweetener)
'like some american lowlife finally' is the wordplay.
'like' becomes 'as'.
'some' becomes 'part' (only part of something).
'american' becomes 'Am' (abbreviation in some contexts).
'finally' suggests the final letters.
The final letter of 'lowlife' is 'e'.
'as'+'part'+'am'+'e'='ASPARTAME'
'offering' acts as a link.
(Other definitions for aspartame that I've seen before include "Strong artificial sweetener -- same apart (anag)" , "What sweetens" , "Sugar substitute" .)