Overrun three American states (6)
I believe the answer is:
invade
'overrun' is the definition.
(I know that overrun can be written as invade)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for invade that I've seen before include "Enter a country by force with an army" , "Forcibly enter (a country)" , "Enter illegally" , "Violate" , "Enter intrusively" .)