Yield a kernel, say (4)
I believe the answer is:
cede
'yield' is the definition.
(I know that yield can be written as cede)
'a kernel say' is the wordplay.
'a kernel' becomes 'seed' (I've seen this before).
'say' indicates a 'sounds like' (homophone) clue.
'seed' is a homophone of 'CEDE'.
(Other definitions for cede that I've seen before include "Surrender or grant" , "Surrender or relinquish possession" , "Surrender or yield" , "21 across" , "Give up (eg, power)" .)