The most common and most likely answer to this crossword clue is the 7 letter word ZINCITE
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The straight part
We know that "Deep red oxide" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deep red oxide = zincite.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 ZINCITE mean?
ZINCITE noun- A yellow, orange or dark-red mineral form of zinc oxide, often also containing small amounts of manganese, with a chemical formula (Zn,Mn2+)O, an important ore of zinc.