This flower may be a big one (7)
I believe the answer is:
begonia
'this flower' is the definition.
(begonia is a kind of flower)
'may be a big one' is the wordplay.
'may be' indicates an anagram.
'a'+'big'+'one'='abigone'
'abigone' anagrammed gives 'BEGONIA'.
(Other definitions for begonia that I've seen before include "Herbaceous plant whose flowers have no petals" , "Common garden plant" , "colourful flower" , "bloomer" , "Tropical plant" .)