Understand, you say, in the main (3)
I believe the answer is:
sea
'main' is the definition.
('sea' can be a synonym of 'main')
'understand you say' is the wordplay.
'understand' becomes 'see' ('see' can be a synonym of 'understand').
'you say' shows a homophone (sound like).
'see' is a homophone of 'SEA'.
'in the' acts as a link.
(Other definitions for sea that I've seen before include "The drink" , "The deep" , "Part of ocean" , "North, say" , "Eg, Aral, Ross, Irish" .)