Showy garden flower (7)
(Other definitions for verbena that I've seen before include "Ornamental plant with showy flowers" , "Vervain (plant)" , "Ornamental pant" , "Herbaceous plant" , "Garden plant" .)
(Other definitions for peony that I've seen before include "Garden flower" , "Plant with large red, white or pink flowers" , "Bloom" , "Herbaceous flower" .)
'showy garden flower' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dahlia that I've seen before include "Plant with tuberous roots and showy coloured flower heads" , "Brightly coloured garden flower with tuberous roots" , "Showy flower from Mexico" , "Peculiar Ali had this flower" , "Showy, big-headed autumn flower" .)