"Aw, shoot!" (4)
(Other definitions for rats that I've seen before include "Curse" , "Rodent pests" , "Oh no" , "(Hunts) rodents - expression of irritation" , "Hamelin's pests" .)
(Other definitions for darn that I've seen before include "curse" , "Mend with 17s - mild expletive" , "Repair holes by sewing" , "Mend with wool using needles" , "Mend (knitted material)" .)
'aw shoot' is the definition.
(I've seen this in another clue)
This is all the clue.