Great deal on American plant (5)
I believe the answer is:
lotus
'plant' is the definition.
'great deal on american' is the wordplay.
'great deal' becomes 'lot' (synonyms).
'on' is a charade indicator (letters next to each other).
'american' becomes 'us' (abbreviation for United States).
'lot'+'us'='LOTUS'
(Other definitions for lotus that I've seen before include "Large water lily" , "Aquatic plant" , "Sacred waterlily" , "Car manufacturer - plant" , "Kind of water lily" .)