Crossing river, pick up plant (4)
I believe the answer is:
bury
'up plant' is the definition.
'bury' can be an answer for 'plant' (burying is a kind of planting). I'm not certain of the 'up' bit.
'crossing river pick' is the wordplay.
'crossing' is an insertion indicator (to cross can mean to go over or around).
'river' becomes 'r'.
'pick' becomes 'buy' (I can't explain this - if you can you should believe this answer much more).
'r' going into 'buy' is 'BURY'.
Can you help me to learn more?
(Other definitions for bury that I've seen before include "Install underground" , "Completely envelop" , "Put under ground" , "Entomb" , "Put underground" .)