Yield what is sown, say (4)
I believe the answer is:
cede
'yield' is the definition.
('cede' can be a synonym of 'yield')
'what is sown say' is the wordplay.
'what is sown' becomes 'seed' (I've seen this before).
'say' shows a homophone (sound like).
'seed' is a homophone of 'CEDE'.
(Other definitions for cede that I've seen before include "Surrender or grant" , "Give away" , "Yield formally" , "Surrender or relinquish possession" , "Relinquish control of" .)