The most common and most likely answer to this crossword clue is the 10 letter word MAINSPRING
I've seen this crossword clue in Independent
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More clues leading to the result MAINSPRING
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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 MAINSPRING mean?
MAINSPRING noun- The principal spring of a clockwork mechanism, which drives it by uncoiling.
- The most important reason for something (Cf. spring "origin of something" (literary) (often in the plural) the springs of her ambition).