Put down pillar with bottom disappearing in river (6)
I believe the answer is:
depose
'put down pillar with bottom' is the definition.
I can't tell whether this definition defines the answer.
'disappearing in river' is the wordplay.
'disappearing' becomes 'pos' (I am not sure about this - if you are sure you should believe this answer much more).
'in' means one lot of letters goes inside another.
'river' becomes 'dee' (River Dee).
'pos' going within 'dee' is 'DEPOSE'.
Can you help me to learn more?
(Other definitions for depose that I've seen before include "Dismiss" , "Oust, overthrow" , "Remove from a high position" , "Remove (from high office)" , "Remove a ruler from office" .)