Marching insect (3)
'marching insect' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for army ant that I've seen before include "Foraging insect" , "Group-foraging insect" , "tropical insect" , "Stinging insect" , "Soldier, perhaps" .)
(Other definitions for ant that I've seen before include "Small busy insect" , "Worker on the hill" , "` `Go to the ... thou sluggard' '" , "Colonial insect?" , "Social worker" .)