The most common and most likely answer to this crossword clue is the 4 letter word JUJU
J
U
J
U
Buddy explains!

The straight part
We know that "Charm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case charm = juju.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 JUJU mean?
JUJU noun- A fetish or charm believed by West Africans to have magical or supernatural powers.
- The magical or supernatural power of such a charm.
- The (usually negative) karmic consequences of an action or behavior.
- A marijuana cigarette; a joint.
- A genre of popular Nigerian music that comes from traditional Yoruba percussion traditions.