The most common and most likely answer to this crossword clue is the 6 letter word EITHER
I've seen this crossword clue in The Guardian
E
I
T
H
E
R
More clues leading to the result EITHER
- Some like it here as one of a twosome
- One or the other solvent absorbs iodine
- One of two theories not so daft
- Tie up that woman, given the choice
- One of two that is returned to the right
- One of two β or one in three, perhaps
- Solvent? I admitted it may introduce an alternative
- Number including unity, one or the other
- Why bother with other resorts I like?
- English 12 β one or the other
Buddy explains!

The straight part
We know that "one or the other" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one or the other = either.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 EITHER mean?
EITHER adverb- (after a negative) As well.
- One or other of two people or things.
- Both, each of two or more.
- Introduces the first of two (or occasionally more) options or possibilities, the second (or last) of which is introduced by βorβ.