Let us take an arbitrary conductor having a weird-shaped cavity inside it. Let $+q$ charge be inserted inside the cavity. The field of $+q$ attracts negative charge & repels positive charge; negative charge accumulates on the surface of the cavity while the induced positive charge accumulates on the outer surface of the conductor till the electric field of the $+q$ charge in the cavity is balanced by the field due to the induced charge inside the conductor. Hence, there exists no electric field inside the conductor due to cancellation of the electric field of the charge residing in the cavity & that formed by the induced charges.
But what happens when an external electric field is applied?
No external fields penetrate the conductor; they are canceled at the outer surface by the induced charge there. Similarly, the field due to charges within the cavity is killed off, for all exterior points, by the induced charge on the inner surface.
How can the induced charge on the outer surface cancel the external field? Hasn't the field of the induced charge been used up for balancing the electric field of the charge in the cavity inside the conductor? How can an used up(I mean to say this field has already been balanced by the field of the cavity-charge inside the conductor) electric field be used to balance another field?? I may be wrong in my sense. Please help.