Crossword Genius

Overrun three American states (6)

Ross

I believe the answer is:

invade

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'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" .)

I've seen this clue in The Independent.
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