The most common and most likely answer to this crossword clue is the 4 plus 3 letter word TWIN BED
I've seen this crossword clue in The Daily Telegraph
T
W
I
N
B
E
D
More clues leading to the result TWIN BED
Buddy explains!

The straight part
We know that "One keeps a sleeping partner at arm’s length" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one keeps a sleeping partner at arm’s length = twin bed.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.