Writer given one American plant (6)
I believe the answer is:
dahlia
'plant' is the definition.
(dahlia is a kind of plant)
'writer given one american' is the wordplay.
'writer' becomes 'dahl' (Roald Dahl).
'given' is a charade indicator (letters next to each other).
'one' becomes 'I' (Roman numeral).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'dahl'+'i'+'a'='DAHLIA'
(Other definitions for dahlia that I've seen before include "Showy garden flower" , "Plant with tuberous roots and showy coloured flower heads" , "Showy garden plant" , "Flower from tuber" , "In bed, root" .)