Plant a big one criminally (7)
I believe the answer is:
begonia
'plant' is the definition.
(begonia is a kind of plant)
'a big one criminally' is the wordplay.
'criminally' indicates an anagram (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 "Herbaceous plant whose flowers have no petals" , "Tropical plant" , "colourful flower" , "Common garden plant" , "bloomer" .)