The most common and most likely answer to this crossword clue is the 5 letter word SCUTE
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The straight part
We know that "protective covering" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case protective covering = scute.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 SCUTE mean?
SCUTE noun- A horny, chitinous, or bony external plate or scale, as on the shell of a turtle or the skin of crocodiles.
- A proneural gene, often associated with achaete, that is required for the formation of many larval and adult sense organs
- A small shield.
- An old French gold coin.