How annoying! (4)
(Other definitions for rats that I've seen before include "Hamelin's pests" , "deserters" , "Curse" , "Treacherous rodents" , "were piped away" .)
(Other definitions for drat that I've seen before include "Blow!" , "that expresses annoyance" , "Bother" , "Expression of annoyance on DART" , "Curse it!" .)