The most common and most likely answer to this crossword clue is the 8 letter word ENDODERM
I've seen this crossword clue in Financial Times
E
N
D
O
D
E
R
M
More clues leading to the result ENDODERM
Buddy explains!

The straight part
We know that "embryonic section" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case embryonic section = endoderm.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 ENDODERM mean?
ENDODERM noun- One of the three tissue layers in the embryo of a metazoan animal. Through development, it will produce the digestive system of the adult.