The most common and most likely answer to this crossword clue is the 4 letter word RSVP
I've seen this crossword clue in The Guardian
R
S
V
P
More clues leading to the result RSVP
Buddy explains!

The straight part
We know that "please reply" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case please reply = rsvp.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 RSVP mean?
RSVP verb- To reply to an invitation.
- (restaurants, events) To reserve a place