Invade in great numbers (7)
I believe the answer is:
overrun
'invade in great numbers' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for overrun that I've seen before include "Go on too long (of a performance)" , "Infest; carry beyond the limit" , "Exceed (allowed cost/time)" , "Exceed time allowed, or spread across and injure (7)" , "Swarm over" .)