Ned's sense is confused with stupidity (9)

I believe the answer is:

denseness

'stupidity' is the definition.
(denseness is a kind of stupidity)

'ned's sense is confused' is the wordplay.
'is confused' indicates anagramming the letters.
'neds'+'sense'='nedssense'
'nedssense' anagrammed gives 'DENSENESS'.

'with' is the link.

(Another definition for denseness that I've seen is " Stupidity".)

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