circular motion is an example of accelerated motion then how can we say that if ''a'' (angular acceleration) = 0 then the motion will be uniform circular motion as the acceleration can never be zero in case of circular motion (because it is an accelerated motion ).

You are actually confusing with two different acceleration. One is linear acceleration (denoted by $a$) and other is angular acceleration (denoted by $\alpha$). For circular motion the linear acceleration can never be zero, but angular acceleration can be zero.

Since in circular motion the direction of linear velocity always changes (and since velocity is a vector quantity), even if the linear speed is constant there will be linear acceleration. But if the circular motion is such that it always trace out equal angle in equal time then angular acceleration is zero.