We have indeterminateness of type
0/0,
i.e. limit for the numerator is
$$\lim_{x \to 0^+}\left(\sin{\left(x \right)} - \tan{\left(x \right)}\right) = 0$$
and limit for the denominator is
$$\lim_{x \to 0^+} x^{3} = 0$$
Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
$$\lim_{x \to 0^+}\left(\frac{\sin{\left(x \right)} - \tan{\left(x \right)}}{x^{3}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(\sin{\left(x \right)} - \tan{\left(x \right)}\right)}{\frac{d}{d x} x^{3}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\cos{\left(x \right)} - \tan^{2}{\left(x \right)} - 1}{3 x^{2}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(\cos{\left(x \right)} - \tan^{2}{\left(x \right)} - 1\right)}{\frac{d}{d x} 3 x^{2}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{- \sin{\left(x \right)} - 2 \tan^{3}{\left(x \right)} - 2 \tan{\left(x \right)}}{6 x}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(- \sin{\left(x \right)} - 2 \tan^{3}{\left(x \right)} - 2 \tan{\left(x \right)}\right)}{\frac{d}{d x} 6 x}\right)$$
=
$$\lim_{x \to 0^+}\left(- \frac{\cos{\left(x \right)}}{6} - \tan^{4}{\left(x \right)} - \frac{4 \tan^{2}{\left(x \right)}}{3} - \frac{1}{3}\right)$$
=
$$\lim_{x \to 0^+}\left(- \frac{\cos{\left(x \right)}}{6} - \tan^{4}{\left(x \right)} - \frac{4 \tan^{2}{\left(x \right)}}{3} - \frac{1}{3}\right)$$
=
$$- \frac{1}{2}$$
It can be seen that we have applied Lopital's rule (we have taken derivatives with respect to the numerator and denominator) 3 time(s)