Hear thieves? Nonsense! (8)
I believe the answer is:
knickers
'nonsense' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'hear thieves?' is the wordplay.
'hear' becomes 'k' (I can't explain this - if you can you should give a lot more credence to this answer).
'thieves?' becomes 'nickers' (nick can mean to steal).
'k'+'nickers'='KNICKERS'
Can you help me to learn more?
(Other definitions for knickers that I've seen before include "Undergarments for women" , "Risk neck (anag.)" , "Mild expletive" , "bother" , "Pants" .)