Hint of weakness one found in metal brace (5)
I believe the answer is:
twain
'brace' is the definition.
('twain' can be a synonym of 'brace')
'hint of weakness one found in metal' is the wordplay.
'hint of' says to take the initial letters.
'one' becomes 'a'.
'found in' means one lot of letters goes inside another.
'metal' becomes 'tin' (tin is a kind of metal**).
The first letter of 'weakness' is 'w'.
'w'+'a'='wa'
'wa' going into 'tin' is 'TWAIN'.
(Other definitions for twain that I've seen before include "Two, formerly" , "Pen name of Samuel Langhorne Clemens, d. 1910)" , "Mark, US writer" , "Mark --, American author" , "Two (arch.)" .)