The most common and most likely answer to this crossword clue is the 5 plus 5 letter word BELLY FLOPS
I've seen this crossword clue in The Daily Telegraph
B
E
L
L
Y
F
L
O
P
S
More clues leading to the result BELLY FLOPS
Buddy explains!
The straight part
We know that "People taking the plunge may have these when entering flat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case people taking the plunge may have these when entering flat = belly flops.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 BELLY FLOPS mean?
BELLY FLOPS noun- A style of diving into a body of water in which the surface impact is made mostly by one's abdomen.