The most common and most likely answer to this crossword clue is the 11 letter word FORTNIGHTLY
I've seen this crossword clue in The Times
F
O
R
T
N
I
G
H
T
L
Y
Buddy explains!

The straight part
We know that "at regular intervals" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case at regular intervals = fortnightly.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 FORTNIGHTLY mean?
FORTNIGHTLY noun- A publication issued fortnightly (once every two weeks).
- Occurring once in a fortnight; once every two weeks
- Once in a fortnight; once every two weeks