As the first in a number of examples or reasons. / As a single given example, often the most obvious or important one and almost certainly enough to sway the argument, of several reasons for something.

present participle of one

simple past tense and past participle of one

In a single attempt. / (theater) Performed downstage between the frontmost pieces of scenery.

present participle of on

simple past tense and past participle of on

(formal) simultaneously, at the same time, all together / (chiefly UK, formal) in agreement

(UK, dated, informal) To be amusing in a slightly challenging way.

Third-person singular simple present indicative form of be a one

simple past tense of be a one