Yield a plant ovule, say (4)
I believe the answer is:
cede
'yield' is the definition.
(I know that yield can be written as cede)
'a plant ovule say' is the wordplay.
'a plant ovule' becomes 'seed' (I've seen this in another clue).
'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 yield" , "Yield formally" , "Relinquish control of" , "Surrender or relinquish possession" , "Give up (power to another)" .)