Mostly sound mistake (4)
I believe the answer is:
goof
'mistake' is the definition.
(I know that mistake can be written as goof)
'mostly sound' is the wordplay.
'mostly' means to remove the last letter.
'sound' becomes 'go off' (go off can mean to make a sound).
'gooff' with its last letter taken away is 'GOOF'.
(Other definitions for goof that I've seen before include "foolish error" , "(Make a) silly mistake" , "In slang, bungle or blunder" , "boob" , "Foolish mistake" .)