The most common and most likely answer to this crossword clue is the 7 letter word HICKORY
H
I
C
K
O
R
Y
Buddy explains!

The straight part
We know that "Nut tree" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nut tree = hickory.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 HICKORY mean?
HICKORY noun- Any of various deciduous hardwood trees of the genus Carya or Annamocarya.
- The wood of these trees.
- Of or pertaining to the hickory tree or its wood.