The most common and most likely answer to this crossword clue is the 11 letter word SKIN-DIVING
I've seen this crossword clue in Financial Times
S
K
I
N
-
D
I
V
I
N
G
Buddy explains!

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 SKIN-DIVING mean?
SKIN-DIVING verb- To swim underwater with equipment.