What is the name of the main feature used in the accompaniment of rock around the clock

The main feature used in the accompaniment of "Rock Around the Clock" is a prominent and driving backbeat.

is the rhythm of rock around the clock's accompaniment off beat

No, the rhythm of "Rock Around the Clock" accompaniment is not offbeat. It has a straightforward, steady 4/4 rhythm with a strong emphasis on the backbeat (on beats 2 and 4). The backbeat is a characteristic element of rock and roll music, providing a driving and energetic feel to the song.