Rudimentary stage (6)
I believe the answer is:
embryo
'rudimentary stage' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for embryo that I've seen before include "Animal in early stage, before birth" , "Beginning of growth" , "Early stage of life" , "Issue conceived" , "Early stage in development of animal or plant" .)