Cryptic crossword clue
With this hammered, you’d secure water-line 8

The most common and most likely answer to this crossword clue is the 8 letter word TREENAIL

I've seen this crossword clue in Financial Times
T
R
E
E
N
A
I
L

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 TREENAIL mean?

TREENAIL noun
  1. A wooden peg or pin used as a fastener.

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