The most common and most likely answer to this crossword clue is the 4 letter word KOOK
K
O
O
K
Buddy explains!
The straight part
We know that "Nut" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nut = kook.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 KOOK mean?
KOOK noun- An eccentric, strange or crazy person.
- (kiteboarding, wakeboarding) A boardsport participant who lacks style or skill; a newbie who acts as if they are better at the sport than they are.