The word basically means 'posh'...ie belonging to the nobs...in the sense of well-dressed and so on. As best I recall, men who had jobs in offices, as 'clerks', were obviously seen as somehow 'grander' than brickies, dustmen or whatever. I believe that is how the nickname 'Nobby' came to be applied to them and, eventually, to anybody with the name, whatever their job. The spellings 'Clark(e)' are just variants on the job-title.