Lab maze runners (4)
I believe the answer is:
rats
'lab maze runners' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rats that I've seen before include "Snoopy's imprecation - vermin" , "Oh no" , "Tells" , "First word of 'The 21 19'" , "Rodent pests" .)