The window opened very easily. Clay needed barely to touch it, and it opened.

From the grammar standpoint, touch is a noun, a singular countable noun, as evidenced by the use of the indefinite article a. "The window opened at one touch", or "at a single touch".

Compare with the expression at a glance:

At a glance: (idiomatic) Upon cursory examination; an abbreviated review. "At a glance it seems that he is a nice guy, but upon digging deeper the truth emerges."

The window opened very easily. Clay needed barely to touch it, and it opened.

The meaning is most likely not literal. Clay might've made a more comprehensive movement than just a single touch to open the window; but that is irrelevant for this narrative; the author needed to stress the fact that it opened easily.

From the grammar standpoint, touch is a noun, a singular countable noun, as evidenced by the use of the indefinite article a. "The window opened at one touch", or "at a single touch".

Compare with the expression at a glance:

At a glance: (idiomatic) Upon cursory examination; an abbreviated review. "At a glance it seems that he is a nice guy, but upon digging deeper the truth emerges."

The window opened very easily. Clay needed barely to touch it, and it opened.

From the grammar standpoint, touch is a noun, a singular countable noun, as evidenced by the use of the indefinite article a. "The window opened at one touch", or "at a single touch".

The window opened very easily. Clay needed barely to touch it, and it opened.

From the grammar standpoint, touch is a noun, a singular countable noun, as evidenced by the use of the indefinite article a. "The window opened at one touch", or "at a single touch".

Compare with the expression at a glance:

At a glance: (idiomatic) Upon cursory examination; an abbreviated review. "At a glance it seems that he is a nice guy, but upon digging deeper the truth emerges."