It is possible to construct a square if only one of its diagonals is given.
Yes
If the length of diagonal of a square is known, we can find out the length of the side of the square. For e.g.
If diagonal length is 'a' in △ABC, using Pythagoras theorem
AB2+ BC2= AC2
x2+x2= a2
2x2= a2
x2= a2/2
x=a/√2
Now, we know that square is a regular quadrilateral. So, all the sides are equal to each other and all the angles are equal to each other. We hence have total of 8 measurements (4 sides + 4 angles). Hence, it is possible to construct a square if only one of its diagonals is known.