Lab warning (4)
I believe the answer is:
bark
'lab warning' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for bark that I've seen before include "Animal sound - layer protecting tree trunk" , "possibly causing alarm in 23 [CATTERY] ?" , "Dog's sound" , "Shout gruffly; part of tree" , "Sound from animal < having part of tree" .)