In Californian city there's a new brown plant (7)

'plant' is the definition.
(lantana is a kind of plant)

'in californian city there's a new brown' is the wordplay.
'in' means one lot of letters goes inside another.
'californian city' becomes 'la'.
'there's' says to put letters next to each other.
'a new' becomes 'an' (I've seen this in other clues**).
'brown' becomes 'tan' (tan is a kind of brown).
'an'+'tan' is 'antan'.
'la' enclosing 'antan' is 'LANTANA'.