The self-focusing of a laser beam due to the Kerr nonlinearity caused by layer deformations in a smectic A liquid crystal (SmA) is considered. It is shown that for a sufficiently high intensity the beam undergoes self-trapping forming a spatial soliton. The same mechanism is responsible for the self-trapping of a surface-guided light wave at the interface of a linear medium and SmA. Numerical estimations show that the new Kerr nonlinearity is approximately 10 times greater than a nonlinearity caused by bulk compression. Self-trapping effect may be observed in smectic A liquid crystal samples of a thickness of 0.01 cm and at a light beam power of an order of magnitude of 0.002 MW.