The most common and most likely answer to this crossword clue is the 10 letter word OCCASIONAL
I've seen this crossword clue in The Times
O
C
C
A
S
I
O
N
A
L
Buddy explains!
The straight part
We know that "Periodic/table of a sort" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case periodic/table of a sort = occasional.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 OCCASIONAL mean?
OCCASIONAL noun- A person who does something only occasionally.
- Occurring or appearing irregularly from time to time, but not often.
- Created for a specific occasion.
- Intended for use as the occasion requires.
- Acting in the indicated role from time to time.