Alone, it damaged hard covering (7)
I believe the answer is:
toenail
'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" .)