Nursery rhyme threesome (4)
I believe the answer is:
mice
'nursery rhyme threesome' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mice that I've seen before include "Computer controls" , "Animals (contrasted with men)" , "Computer devices; black eyes" , "people lacking character" , "Domestic rodents" .)