Crazy blunders involving knucklehead (7)
I believe the answer is:
bonkers
'crazy' is the definition.
(I've seen this before)
'blunders involving knucklehead' is the wordplay.
'blunders' becomes 'boners' ('boner' can be a synonym of 'blunder').
'involving' is an insertion indicator.
'knucklehead' becomes 'k' (head letter of 'knuckle').
'boners' enclosing 'k' is 'BONKERS'.
(Other definitions for bonkers that I've seen before include "bats" , "Out to lunch" , "Nuts" , "They could reproduce" , "Barmy, crackers" .)