The most common and most likely answer to this crossword clue is the 4 plus 4 letter word HEAD BUTT
I've seen this crossword clue in Financial Times
H
E
A
D
B
U
T
T
More clues leading to the result HEAD BUTT
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 = head butt.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.
Crossword Answer definitions
HEAD BUTT noun- A sharp blow delivered by driving the head into an opponent or object, generally by lowering the head and charging forward or by rapidly tilting the head backward and then forward.
- To deliver a sharp blow by driving the head into an opponent or object.