The deep note sounded (3)
I believe the answer is:
sea
'the deep' is the definition.
(I've seen this before)
'note sounded' is the wordplay.
'note' becomes 'si' (I've seen this in other clues).
'sounded' shows a homophone (sound like).
'si' sounds like 'SEA'.
(Other definitions for sea that I've seen before include "All at . . . - totally confused" , "host of 1d [ADVENTURE]" , "Eg, Aral, Ross, Irish" , "E.g. Yellow, Red area" , "eg Galilee" .)