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