The most common and most likely answer to this crossword clue is the 6 letter word ANLAGE
A
N
L
A
G
E
Buddy explains!
The straight part
We know that "First sign of organ" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case first sign of organ = anlage.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 ANLAGE mean?
ANLAGE noun- A primordium, the initial clustering of embryonic cells from which a body part develops.
- An allele, a specific version of a gene (as used by Gregor Mendel).
- Temperament, the predominant personality type.