Answer
$-\dfrac {\left( 1-\cos x\right) \left( \cos \left( x-\sin x\right) \right) }{\sin ^{2}\left( x-\sin x\right) }$
Work Step by Step
$y=\dfrac {1}{\sin \left( x-\sin x\right) }\Rightarrow y'=-\dfrac {\left( \cos \left( x-\sin x\right) \right) \times \left( \dfrac {d}{dx}\left( x-\sin x\right) \right) }{\sin ^{2}\left( x-\sin x\right) }=-\dfrac {\left( 1-\cos x\right) \left( \cos \left( x-\sin x\right) \right) }{\sin ^{2}\left( x-\sin x\right) }$
