The most common and most likely answer to this crossword clue is the 6 letter word GNOMON
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Buddy explains!
The straight part
We know that "sundial rod" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sundial rod = gnomon.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 GNOMON mean?
GNOMON noun- An object such as a pillar or a rod that is used to tell time by the shadow it casts when the sun shines on it, especially the pointer on a sundial.
- An object such as a pillar used by an observer to calculate the meridian altitude of the sun (that is, the altitude of the sun when it reaches the observer's meridian), for the purpose of determining the observer's latitude.
- The index of the hour circle of a globe.
- A plane figure formed by removing a parallelogram from a corner of a larger parallelogram.
- (by extension) A number representing the increment between two figurate numbers.