The most common and most likely answer to this crossword clue is the 9 letter word ETERNISES
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More clues leading to the result ETERNISES
Buddy explains!
The straight part
We know that "causes to last forever" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case causes to last forever = eternises.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 ETERNISES mean?
ETERNISES verb- To make or render eternal.
- To prolong indefinitely.
- To immortalize; to make eternally famous.