Leonard back behind bog plant (6)
I believe the answer is:
fennel
'plant' is the definition.
(fennel is a kind of plant)
'leonard back behind bog' is the wordplay.
'leonard' becomes 'len' (I've seen this in other clues).
'back' says the letters should be written backwards.
'behind' says to put letters next to each other.
'bog' becomes 'fen' (**).
'len' in reverse letter order is 'nel'.
'nel' put after 'fen' is 'FENNEL'.
(Other definitions for fennel that I've seen before include "Umbelliferous herb" , "type of seasoning" , "Herb Len raised behind the fen" , "Plant used for flavouring, such as finocchio" , "...and an aromatic flavourful herb" .)