The most common and most likely answer to this crossword clue is the 6 letter word TESTES
T
E
S
T
E
S
More clues leading to the result TESTES
- Investigations involving lower part of reproductive glands
- Having eaten the last of entrée, try special nuts
- Balls: “Try drugs”
- Some privates switched positions
- Nuts put TV upside down
- Check tip of electrode on top of sperm-producing glands
- Part of tennis match repeatedly knocked back balls
- Pair of males succeeded on challenge number five
- Check-up incomplete; repeated for an organ
Buddy explains!

The straight part
We know that "organ" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case organ = testes.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 TESTES mean?
TESTES noun- A witness.
- The witnessing or concluding clause, duty attached; said of a writ, deed, etc.
- A testicle of a vertebrate.
- An analogous gland in invertebrates such as the hydra.