Fodder plant (7)
'fodder plant' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lucerne that I've seen before include "Swiss city - alfalfa" , "Clover-like plant used for fodder" , "Swiss city/lake" , "Swiss city; cattle feed" , "Fodder plant - lake" .)
I believe this is a double definition.
'fodder' is the first definition.
(alfalfa is a kind of fodder)
'plant' is the second definition.
(alfalfa is a kind of plant)
(Other definitions for alfalfa that I've seen before include "food plant" , "cattle feed" , "Plant used as fodder" , "odder plant" , "Lucerne, a fodder plant" .)