Flood in lower position to involve river (5)
I believe the answer is:
drown
'flood' is the definition.
(I've seen this before)
'lower position to involve river' is the wordplay.
'lower position' becomes 'down' (I am not sure about this - if you are sure you should believe this answer much more).
'to involve' means one lot of letters goes inside another (inserted letters get involved).
'river' becomes 'r' (abbreviation for river).
'down' going around 'r' is 'DROWN'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for drown that I've seen before include "Swamp" , "Immerse (fatally?)" , "Submerge, maybe fatally" , "Die in water" , "Suffocate in liquid" .)