Small arachnids
I believe the answer is:
mites
'small arachnids' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mites that I've seen before include "Small arachnids" , "Minute creatures that infest animals and food" , "little children" , "Minute arachnids that infest animals and stored foods" , "Widows' contributions?" .)