The most common and most likely answer to this crossword clue is the 5 letter word PSST!
I've seen this crossword clue in The Guardian
P
S
S
T
!
More clues leading to the result PSST!
Buddy explains!

The straight part
We know that "Take notice /Take notice" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case take notice /take notice = psst!.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 PSST! mean?
PSST! verb- To say psst, to get someone's attention or to communicate with them.
- Implies that the speaker is sending secret or whispered information to another person.