The first thing to note is that a_ is a short hand notation. It is almost always helpful to look at the FullForm when you are trying to write patterns. When you do that with a_ you will notice that it is interpreted as
Pattern[a, Blank[]]
From the docs, this can be written as
a: _
This provides a way to name repeated sequences, e.g.
a : Repeated[_, {k}] (* where k has to be a positive integer *)
c : {a_, b_}
c : {a_, b : Repeated[_, {k}]}
or any other pattern you wish.
As an additional note, when using a in
a : Repeated[_, {k}]
it is a Sequence, and a typical method for manipulating it is to place it inside of a List, e.g.
(* the simplest example *)
f[a : Repeated[_, {k}]] := {a}