These knickers are rubbish! (5)
I believe the answer is:
pants
This clue is a double definition.
'these knickers' is the first definition.
(synonyms)
'rubbish' is the second definition.
(informal term meaning poor-quality)
'are' acts as a link.
(Other definitions for pants that I've seen before include "Undergarments; rubbish" , "Breathes heavily in trousers" , "Garment; rubbish" , "awful according to modern youngsters" , "See 17" .)