Septahedron
The shape you're looking for is "seesaw." It has 11 letters, possesses at least one line of symmetry, and starts with the letter "s." A seesaw can be symmetrical along its central axis.
sausage
a isosceles triangle has at least 1 line of symmetry but only has 9 letters
The letters S, N, Z, for example.
The term you're looking for is "asymmetrical." However, "asymmetrical" actually means lacking symmetry, so it does not fit your criteria. A shape with at least one line of symmetry could be described as "symmetrical," but that only has 10 letters. If you have another specific term or context in mind, please clarify!
a symmetrical shape
An isosceles triangle
A square
The answer is a septahedron, 11 letter answer
sausage
a isosceles triangle has at least 1 line of symmetry but only has 9 letters
a shape with a line of symmetry
The letters S, N, Z, for example.
a bottomhead
semicircles
The 2D shape which has one line of symmetry is the trapezium.
An isosceles triangle has one line of symmetry