The most common and most likely answer to this crossword clue is the 6 letter word ALDRIN
I've seen this crossword clue in The Guardian
A
L
D
R
I
N
Buddy explains!

The straight part
We know that "Astronaut/Astronaut" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case astronaut/astronaut = aldrin.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 ALDRIN mean?
ALDRIN noun- An insecticide and persistent organic pollutant containing a naphthalene-derived compound.