cock of the walk = a man who acts as if he is more fashionable or important than other people
e.g. She acts like the cock of the walk in the college.
So you can replace it with its meaning.
e.g. She acts like she is more important than others.
She acts as if she is the king of the world
She acts like she is the goddess of fashion
Cock of the walk is a very seldom used phrase.