The most common and most likely answer to this crossword clue is the 5 letter word BLOOP
I've seen this crossword clue in Independent
B
L
O
O
P
More clues leading to the result BLOOP
Buddy explains!

The straight part
We know that "Gentle hit/Billy Liar/s" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gentle hit/billy liar/s = bloop.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 BLOOP mean?
BLOOP noun- A low-pitched beeping sound.
- To make a hit just beyond the infield.
- To produce a low-pitched beeping sound.
- To cover up splices in a soundtrack tape to eliminate the unwanted noise they may produce.