Plant in river in the past blocking lake (8)
I believe the answer is:
tarragon
'plant in' is the definition.
'tarragon' can be an answer for 'plant' (I've seen this before). I'm unsure of the 'in' bit.
'river in the past blocking lake' is the wordplay.
'river' becomes 'r'.
'in the past' becomes 'ago' ('ago' can be a synonym of 'in the past').
'blocking' indicates putting letters inside.
'lake' becomes 'tarn' (tarn is a kind of lake**).
'r'+'ago'='rago'
'rago' going into 'tarn' is 'TARRAGON'.
Can you help me to learn more?
(Other definitions for tarragon that I've seen before include "cooking ingredient" , "leaves" , "(Aromatic plant producing) herb" , "seasoning" , "cooks may use it" .)