The most common and most likely answer to this crossword clue is the 8 letter word TETRAGON
I've seen this crossword clue in The Times
T
E
T
R
A
G
O
N
More clues leading to the result TETRAGON
Buddy explains!

The straight part
We know that "shape" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shape = tetragon.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 TETRAGON mean?
TETRAGON noun- Quadrilateral.
- An aspect of two planets with regard to the Earth when they are distant from each other ninety degrees, or a quarter-circle.