Nearly fool heartless Bellow by acting stupid (6)
I believe the answer is:
idiocy
'acting stupid' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'nearly fool heartless bellow' is the wordplay.
'nearly' means to remove the last letter.
'fool' becomes 'idiot' (I've seen this before).
'heartless bellow' becomes 'cy' (I can't explain this - if you can you should give a lot more credence to this answer).
'idiot' with its final letter taken away is 'idio'.
'idio'+'cy'='IDIOCY'
'by' acts as a link.
Can you help me to learn more?
(Other definitions for idiocy that I've seen before include "Extreme silliness" , "Complete folly" , "Extreme foolishness" , "Abject stupidity" , "Foolish act" .)