The most common and most likely answer to this crossword clue is the 7 letter word RUSTING
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Buddy explains!
The straight part
We know that "becoming dull" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case becoming dull = rusting.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 RUSTING mean?
RUSTING verb- To oxidize, especially of iron or steel.
- To cause to oxidize.
- To be affected with the parasitic fungus called rust.
- To (cause to) degenerate in idleness; to make or become dull or impaired by inaction.
- The development of rust on a surface.