Picnic invaders (4)
I believe the answer is:
ants
'picnic invaders' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ants that I've seen before include "Tiny insects that live in colonies" , "They live in a formicary" , "Termites" , "Inhabitants of a formicary" , "Small social insects" .)