Translation:I will be in work at eight o'clock.
I'm not sure what you mean, the two sentences I can see are "Bydda i yn y gwaith am wyth o'r gloch." and "I will be in work for eight o'cloc". Am in this instance is used to mean for. I would also say "I will be at work by eight o'clock" as "Bydda i yn y gwaith erbyn wyth o'r gloch".