The most common and most likely answer to this crossword clue is the 6 plus 6 letter word HIGHLY STRUNG
I've seen this crossword clue in The Sunday Telegraph
H
I
G
H
L
Y
S
T
R
U
N
G
More clues leading to the result HIGHLY STRUNG
Buddy explains!
The straight part
We know that "Like support for tightrope artist/ very tense" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like support for tightrope artist/ very tense = highly strung.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.