A great deal on American water lily (5)
I believe the answer is:
lotus
'water lily' is the definition.
(lotus is a kind of water lily)
'a great deal on american' is the wordplay.
'a 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 "Type of water lily" , "Exotic flower" , "Water lily - fruit inducing forgetfulness" , "Type of water-lily" , "Mythological fruit - lily" .)