The most common and most likely answer to this crossword clue is the 8 letter word PROOFING
I've seen this crossword clue in The Daily Telegraph
P
R
O
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F
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More clues leading to the result PROOFING
Buddy explains!

The straight part
We know that "Making watertight" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case making watertight = proofing.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 PROOFING mean?
PROOFING verb- To proofread.
- To make resistant, especially to water.
- To allow yeast-containing dough to rise.
- To test the activeness of yeast.
- A step in creating yeast breads and baked goods, during which the yeast is allowed to leaven the dough.