No term for mediocrity of stupidity (6)
I believe the answer is:
idiocy
'stupidity' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for idiocy that I've seen before include "Extreme foolishness" , "Extreme silliness" , "Great folly" , "Predatory seabird" , "Foolish action or behaviour" .)