The most common and most likely answer to this crossword clue is the 10 letter word BLACKENING
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Buddy explains!

The straight part
We know that "growing more opaque" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case growing more opaque = blackening.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 BLACKENING mean?
BLACKENING verb- (causative) To cause to be or become black.
- To become black.
- (causative) To make dirty.
- To defame or sully.
- To cook (meat or fish) by coating with pepper, etc., and quickly searing in a hot pan.
- The act of turning black in colour; a black stain or mark.