Main sound effects agonising initially (3)
I believe the answer is:
sea
'main' is the definition.
('sea' can be a synonym of 'main')
'sound effects agonising initially' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'sound effects agonising' is 'SEA'.
(Other definitions for sea that I've seen before include "Large body of salt water" , "& 14 Ozone" , "Lesser ocean" , "Caspian, e.g." , "Body of saltwater" .)