The most common and most likely answer to this crossword clue is the 5 plus 5 letter word SPACE CADET
S
P
A
C
E
C
A
D
E
T
Buddy explains!
The straight part
We know that "Trainee astronaut/who keeps taking out-of-this-world trips" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trainee astronaut/who keeps taking out-of-this-world trips = space cadet.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.