Occupy in large numbers (7)
I believe the answer is:
overrun
'occupy in large numbers' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for overrun that I've seen before include "Infest; carry beyond the limit" , "Invade - exceed time limit" , "Go beyond (time/budget)" , "Exceed (cost estimate)" , "Occupy (a place) in large numbers" .)