Thick-headed, thoughtless rubbish (5)
I believe the answer is:
trash
'rubbish' is the definition.
(I know that rubbish is a class)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for trash that I've seen before include "Worthless items" , "Rubbish, refuse" , "Vandalise" , "Rubbish, litter" , "bunk" .)