The most common and most likely answer to this crossword clue is the 8 letter word NUTATION
I've seen this crossword clue in The Guardian
N
U
T
A
T
I
O
N
Buddy explains!

The straight part
We know that "Nodding" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nodding = nutation.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 NUTATION mean?
NUTATION noun- A bobbing motion that accompanies the precession of a spinning rigid body.
- A nodding motion (of the head etc.).
- Any of several irregularities in the precession of the equinoxes caused by varying torque applied to the Earth by the Sun and the Moon.
- The circular motion of the tip of a growing shoot.