Encroach on (6)
I believe the answer is:
invade
'encroach on' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for invade that I've seen before include "Penetrate other country with armed forces" , "Enter by force so as to conquer" , "infect" , "Enter another's territory" , "Intrude on" .)