Main letter read out (3)
I believe the answer is:
sea
'main' is the definition.
(synonyms)
'letter read out' is the wordplay.
'letter' becomes 'see' (possible spelling of 'C' according to Chambers).
'read out' shows a homophone (sound like).
'see' is a homophone of 'SEA'.
(Other definitions for sea that I've seen before include "Cause of flooding" , "E.g. Baltic" , "Lesser ocean" , "that's easily reached from Dover?" , "Marine" .)