The most common and most likely answer to this crossword clue is the 6 letter word WITHIN
I've seen this crossword clue in The Sunday Telegraph
W
I
T
H
I
N
Buddy explains!

The straight part
We know that "Not beyond" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not beyond = within.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 WITHIN mean?
WITHIN adjective- In the context of which the present document or ruling is made.
- In or into the interior; inside.
- In the inner part, spatially; physically inside.
- In the scope or range of.
- Before the specified duration ends.