In The Three Clerks Trollope does give some detail:
she was enabled to assume that covering for her
head in which her soul most delighted. It was a Tom and Jerry hat turned up at the sides, with a short but knowing feather, velvet trimmings, and a steel buckle blinking brightly in the noonday
sun.
I'm not sure what Tom and Jerry had to do with the hat apart from the name! And just what is a knowing feather?