Message that’s loud a mark of error (3)
I believe the answer is:
fax
'message' is the definition.
'loud a mark of error' is the wordplay.
'loud' becomes 'f' (A musical term**).
'mark of error' becomes 'x' (mark placed next to error, opposite of a tick).
'f'+'a'+'x'='FAX'
'that's' acts as a link.
(Other definitions for fax that I've seen before include "Transmit document" , "Transmission of exact copies of documents electronically" , "Electronic transmission" , "Modern method of transmitting printed matter by phone" , "Means of communication" .)