The most common and most likely answer to this crossword clue is the 5 letter word HALLO
I've seen this crossword clue in Financial Times
H
A
L
L
O
More clues leading to the result HALLO
Buddy explains!

The straight part
We know that "greeting" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case greeting = hallo.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 HALLO mean?
HALLO noun- The cry "hallo!"
- A shout of exultation.
- To shout, or to call with a loud voice.
- To chase while shouting "hallo!"
- To cry "hallo" (to someone).
- To shout (something).
- A cry of surprise.
- A greeting (salutation) said when meeting someone or acknowledging someone’s arrival or presence.
- A greeting used when answering the telephone.
- A call for response if it is not clear if anyone is present or listening, or if a telephone conversation may have been disconnected.
- Used sarcastically to imply that the person addressed or referred to has done something the speaker or writer considers to be foolish.
- An expression of puzzlement or discovery.