"Where is a clock?"
Translation:Gdzie jest zegar?
Well, "Gdzie to zegar" makes no sense, because "to" is neuter and "zegar" is masculine. "to" also cannot be a dummy pronoun here (as in "To jest zegar" = "This/That/It is a clock"), because it also doesn't suit the sentence at all.
If you were thinking about "to" as in "Kot to zwierzę", 'to' doesn't exactly mean 'is', it is more like a '=' sign. That's why you absolutely can't use it here.
"ten" was suggested because it's the closest accepted answer. As it means "this", it's an interpretation of "the" (Where is the clock, Where is this clock).
In the questions about 'where is something' you can omit "jest" (Gdzie [jest/] [ten/] zegar?), but it may be considered impolite, so I do not recommend it.