Squeal or squealer (3)
I believe the answer is:
rat
'squeal or squealer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rat that I've seen before include "Rodent; informer (colloq.)" , "shopper" , "one not going down with ship" , "Betray like a rodent" , "Sewer rodent" .)