William Sydney Clack (31 March 1910-29 January 1984) was a Conservative Party local politician.
Born in Fulham, he was the son of a baker. He moved to Pinner in the Borough of Harrow, Middlesex, and in 1955 was elected to fill a casual vacancy on the borough council. He remained a councillor when the municipal borough became the London Borough of Harrow in 1965, later becoming an alderman and Mayor of Harrow for 1969-70.
In 1978 he married Jill Elizabeth Gould, who was also a member of Harrow Borough Council and the GLC.