Had returned and left one a plant (6)
I believe the answer is:
dahlia
'plant' is the definition.
(dahlia is a kind of plant)
'had returned and left one a' is the wordplay.
'returned' is a reversal indicator.
'and' is a charade indicator (letters next to each other).
'left' becomes 'l' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'had' reversed gives 'dah'.
'dah'+'l'+'i'+'a'='DAHLIA'
(Other definitions for dahlia that I've seen before include "Showy flower from Mexico" , "Tuberous garden plant" , "Lia had this flower, strangely" , "Showy herbaceous perennial" , "'Hi, a lad has the flower (6)'" .)