Answer
$-\dfrac{2 \sqrt{3}}{3}$
Work Step by Step
The cosecant is an odd function so
$\csc{(- \theta)} = - \csc{\theta}$
Thus,
$\csc{\left(-\dfrac{\pi}{3} \right)} = - \csc{\left(\dfrac{\pi}{3} \right)} =\boxed{-\dfrac{2 \sqrt{3}}{3}}$