Mild expletive — vermin (4)
I believe the answer is:
rats
I believe this clue is a double definition.
'mild expletive' is the first definition.
(I've seen this in another clue)
'vermin' is the second definition.
(I've seen this before)
(Other definitions for rats that I've seen before include "Annoyed expression" , "Damn" , "deserters" , "Snoopy's imprecation - vermin" , "Curse" .)