Writer popular with children — one with a flower (6)
I believe the answer is:
dahlia
'flower' is the definition.
(I know that dahlia is a type of flower)
'writer popular with children one with a' is the wordplay.
'writer popular with children' becomes 'Dahl' (Roald Dahl).
'one' becomes 'i' (Roman numeral).
'with' is a charade indicator (letters next to each other).
'dahl'+'i'+'a'='DAHLIA'
(Other definitions for dahlia that I've seen before include "Tuberous plant with showy flowers" , "Flower -- pale violet or amethyst colour" , "Suggestive of soil" , "Flower from tuber" , "Brightly coloured garden flower with tuberous roots" .)