Crossword Genius

Alone, it damaged hard covering (7)

Ross

I believe the answer is:

toenail

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'hard covering' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'alone it damaged' is the wordplay.
'damaged' indicates anagramming the letters.
'alone'+'it'='aloneit'
'aloneit' with letters rearranged gives 'TOENAIL'.

Can you help me to learn more?

(Other definitions for toenail that I've seen before include "One of ten" , "Hard plate on part of foot" , "Part of body perhaps painted" , "Digital cover-- 21 across (anag)" , "part of leg" .)

I've seen this clue in The Daily Express.
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