Main witness by the sound of it (3)
I believe the answer is:
sea
'main' is the definition.
(the main is an old name for the sea)
'witness by the sound of it' is the wordplay.
'witness' becomes 'see' (synonyms).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'see' is a homophone of 'SEA'.
(Other definitions for sea that I've seen before include "that's easily reached from Dover?" , "North for one" , "E.g Aral or Weddell" , "Body of saltwater" , "Large body of salt water" .)