Tease American over sweetheart (5)
I believe the answer is:
sugar
'sweetheart' is the definition.
(thesaurus)
'tease american over' is the wordplay.
'tease' becomes 'rag' (synonyms).
'american' becomes 'us' (abbreviation for United States).
'over' shows that the letters should be reversed in order.
'rag'+'us'='ragus'
'ragus' reversed gives 'SUGAR'.
(Other definitions for sugar that I've seen before include "Plantation product" , "flattery" , "Sweet white substance" , "rotter!" , "Common sweetener" .)