Swarming insect (6)
(Other definitions for locus that I've seen before include "insect" , "Place of occurrence" , "place in court matters" , "Particular place something is found" , "a result of a certain formula" .)
'swarming insect' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for locust that I've seen before include "Voracious insect in dreaded swarms" , "plaguey type" , "Voracious insect, travels in swarms" , "Destructive migratory insect" , "Swarming, devouring, insect" .)