Plant bay bordering lake and river (6)
I believe the answer is:
clover
'plant' is the definition.
(clover is a kind of plant)
'bay bordering lake and river' is the wordplay.
'bay' becomes 'cove' (I've seen this before).
'bordering' means one lot of letters goes inside another.
'lake' becomes 'L' (geographical abbreviation).
'and' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation for river).
'cove' going around 'l' is 'clove'.
'clove'+'r'='CLOVER'
(Other definitions for clover that I've seen before include "Common trefoil" , "Trifolium" , "Wild three-leafed plant" , "Pasture plant" , "A four - leafed one is for luck (6)" .)