Hill-building insects (4)
I believe the answer is:
ants
'hill-building insects' 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" , "24 [INSECT] s" , "Small social insects" , "Inhabitants of a formicary" , "workers found returning" .)