Flower could become a big one (7)
I believe the answer is:
begonia
'flower' is the definition.
(I know that Begonia is a type of flower)
'become a big one' is the wordplay.
'become' is an anagram indicator.
'a'+'big'+'one'='abigone'
'abigone' with letters rearranged gives 'BEGONIA'.
'could' is the link.
(Other definitions for begonia that I've seen before include "colourful flower" , "Tropical plant" , "bloomer" , "Common garden plant" , "Herbaceous plant whose flowers have no petals" .)