Resentful as the chap who's out of the race? (5)
I believe the answer is:
ratty
But I can't explain it at all! Could you help me understand?
(Other definitions for ratty that I've seen before include "Mole's friend (The Wind in the Willows)" , "Short" , "Waspish" , "Cross - character in "The Wind in the Willows"" , "Characteristic of pest" .)