Hello. You did not underline any clause in the sentence above, which makes it impossible for this question to be answered. However, I will try to help you in the best possible way.
If the underlined clause is "tis going" it means that the speaker of the sentence is indicating that the clock is working.
If the underlined clause is "it strikes well and hearty" it means that the watch is working very well, vigorously and correctly. This is because the words used give the feeling of something that is healthy and that has vigor and disposition for any activity.