Make fun of American for rejecting sweetener (5)
I believe the answer is:
sugar
'sweetener' is the definition.
(sugar is a kind of sweetener)
'make fun of american for rejecting' is the wordplay.
'make fun of' becomes 'rag' (to rag on someone is to make fun of them).
'american' becomes 'us' (abbreviation for United States).
'for rejecting' is a reversal indicator.
'rag'+'us'='ragus'
'ragus' written backwards gives 'SUGAR'.
(Other definitions for sugar that I've seen before include "Damn" , "--- Ray Robinson, boxer" , "flattery" , "Sweet white substance" , "Plantation product" .)