Extremes of resentment about us? Rot! (4)
I believe the answer is:
rust
'rot' is the definition.
(thesaurus)
'extremes of resentment about us?' is the wordplay.
'extremes of' says to hollow out the word (remove centre letters) (only the letters at the extremes of the word).
'about' is an insertion indicator.
'resentment' with its middle taken out is 'rt'.
'rt' going around 'us' is 'RUST'.
(Other definitions for rust that I've seen before include "Oxidised iron" , "deteriorate" , "A fungus; a corrosion" , "Fungal disease of plants" , "Reddish oxide coating on iron" .)