Immature egg (5)
I believe the answer is:
ovule
'immature egg' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ovule that I've seen before include "Small body containing the female germ cell of a plant" , "Egg cell" , "Immature ovum" , "Egg-cell in a plant" , "Undeveloped seed" .)