Change the following sentence into passive voice:
Somebody will clean the office tomorrow.

When the subject does the action, the statement is in the active voice. 
The subject of the verb "will clean" in the preceding sentence is "someone." The subject is omitted in the passive voice form when the subject pronouns are vague, such as somebody, nobody, anybody, they, etc.
The answer is: The office will be cleaned tomorrow when the above sentence is transformed to passive and the subject "someone" is omitted.