The most common and most likely answer to this crossword clue is the 5 letter word DUNCE
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More clues leading to the result DUNCE
- Caught hiding in sand? I never learn
- Goose departs before cat drops round
- Slow learner caught dipping into Herbert’s sci-fi classic
- Centrist's head in the sand — one never learns
- Uneducated type in drably coloured church …
- Dutch pawnbroker ripped off lecturer; he’s a slow learner!
- Moron expected sandwiches, not cheese, to start with
- Fool about, getting covered by mound of sand
- Fool‘s caught trapped in sand
- Charlie caught in nude romps
Buddy explains!

The straight part
We know that "Knucklehead" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case knucklehead = dunce.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DUNCE mean?
DUNCE noun- An unintelligent person.