Enter en masse (4,2)
'enter en masse' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Another definition for pile in that I've seen is " board in disorder".)
(Other definitions for invade that I've seen before include "Enter illegally" , "Enter intrusively" , "Enter a country by force with an army" , "Enter by force so as to conquer" , "Enter with armed forces to attack" .)