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

The straight part
We know that "Expressing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case expressing = reflecting.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 REFLECTING mean?
REFLECTING verb- To bend back (light, etc.) from a surface.
- To be bent back (light, etc.) from a surface.
- To mirror, or show the image of something.
- To be mirrored.
- To agree with; to closely follow.
- To give evidence of someone's or something's character etc.
- To think seriously; to ponder or consider.
- An instance of reflection
- That has been reflected (of light, sound etc.); undergoing reflection.
- That reflects.
- Having a component that reflects light.
- Given to introspection; thoughtful, reflective.
- Insulting, disparaging.