The most common and most likely answer to this crossword clue is the 9 letter word OSSIFYING
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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.
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What does OSSIFYING mean?
OSSIFYING verb- To transform (or cause to transform) from a softer animal substance into bone; particularly the processes of growth in humans and animals.
- (animate) To become (or cause to become) inflexible and rigid in habits or opinions.
- (inanimate) To grow (or cause to grow) formulaic and permanent.
- To calcify.