The most common and most likely answer to this crossword clue is the 9 letter word EMBRYONIC
I've seen this crossword clue in Financial Times
E
M
B
R
Y
O
N
I
C
Buddy explains!

The straight part
We know that "Rudimentary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rudimentary = embryonic.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does EMBRYONIC mean?
EMBRYONIC adjective- Of or relating to an embryo.
- Of a project, etc: very new and still evolving; yet to reach its full potential.