Herb from marsh Leonard brought back (6)
I believe the answer is:
fennel
'herb' is the definition.
(fennel is a kind of herb)
'marsh leonard brought back' is the wordplay.
'marsh' becomes 'fen' ('fen' can be a synonym of 'marsh').
'leonard' becomes 'len' (I've seen this in other clues).
'brought back' says the letters should be written in reverse.
'len' backwards is 'nel'.
'fen'+'nel'='FENNEL'
'from' is the link.
(Other definitions for fennel that I've seen before include "Plant used for seasoning" , "type of seasoning" , "Edible plant" , "Herb Len raised behind the fen" , "Culinary plant" .)