The most common and most likely answer to this crossword clue is the 6 letter word SPACER
S
P
A
C
E
R
More clues leading to the result SPACER
- Long-distance traveller goes through again on return trip
- Material put in gap for astronaut
- Astronaut in secure capsule ascending
- Astronaut will make Mars finally, given speed, right?
- It staggers and goes over backwards
- Senior accepts rate for orthodontic gadget
- Bar special champion, right?
- Short summaries about an aid for laying tiles
- Summary’s offered up: it keeps items separate
- Being held up, goes over what creates a gap
Buddy explains!

The straight part
We know that "orthodontic gadget" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case orthodontic gadget = spacer.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.