Sound of final bell disturbed one within (10)
I believe the answer is:
infallible
'sound' is the definition.
(both can mean solid or correct)
'final bell disturbed one within' is the wordplay.
'disturbed' indicates anagramming the letters.
'one' becomes 'I' (Roman numeral).
'within' means one lot of letters goes inside another.
'final'+'bell'='finalbell'
'finalbell' anagrammed gives 'infallble'.
'infallble' enclosing 'i' is 'INFALLIBLE'.
'of' acts as a link.
(Other definitions for infallible that I've seen before include "Incapable of error" , "faultless" , "Perfect" , "Unable to make a mistake" , "Always right" .)