Lack of feeling shows stupidity (13)
I believe the answer is:
senselessness
'stupidity' is the definition.
(I've seen this before)
'lack of feeling' is the wordplay.
I cannot really see how this works, but
'feeling' could be 'sense' (I've seen this before) and 'sense' is present in the answer.
This explanation may well be incorrect...
'shows' acts as a link.
Can you help me to learn more?
(Other definitions for senselessness that I've seen before include "Stupidity" , "Inanity" .)