The most common and most likely answer to this crossword clue is the 9 letter word NEPTUNIUM
N
E
P
T
U
N
I
U
M
Buddy explains!
The straight part
We know that "on table periodically" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case on table periodically = neptunium.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 NEPTUNIUM mean?
NEPTUNIUM noun- The transuranic chemical element with atomic number 93 and symbol Np.