Cryptic crossword clue
Gents swimming across river subsequently get tougher 10

The most common and most likely answer to this crossword clue is the 10 letter word STRENGTHEN

I've seen this crossword clue in The Daily Telegraph
S
T
R
E
N
G
T
H
E
N


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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 STRENGTHEN mean?

STRENGTHEN verb
  1. To make strong or stronger; to add strength to; to increase the strength of; to fortify.
  2. To empower; to give moral strength to; to encourage; to enhearten.
  3. To augment; to improve; to intensify.
  4. To grow strong or stronger.

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