What's in this shoe? I don't believe it! (2,4)
I believe the answer is:
my foot
'what's' is the definition.
Both the definition and answer are interjections.
Maybe you can see an association between them that I can't see?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for my foot that I've seen before include "Nonsense!" , "I just don't believe it!" , "I've some nails here?" , "rubbish!" , "Interjection expressing contemptuous disbelief" .)