The most common and most likely answer to this crossword clue is the 4 letter word WEDS
I've seen this crossword clue in The Daily Telegraph
W
E
D
S
More clues leading to the result WEDS
Buddy explains!

The straight part
We know that "takes the plunge" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case takes the plunge = weds.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 WEDS mean?
WEDS verb- To perform the marriage ceremony for; to join in matrimony.
- To take as one's spouse.
- To take a spouse.
- To join or commit to, more or less permanently, as if in marriage.
- To take to oneself and support; to espouse.
- To wager, stake, bet, place a bet, make a wager.