Leading rating (or leading rate) is the most senior of the junior rates in the Royal Navy. It is equal in status to corporal, although for a long time was officially junior to that rank. The rate was introduced under the authority of Admiralty Circular No. 121 of 14 June 1853.
Leading ratings are normally addressed as "Leading Hand" or using their branch title e.g. Leading Seaman, Leading Regulator etc.
The insignia worn by leading rates is a single fouled anchor, leading to the slang term killick or hooky used in reference to this rate.
References
Leading rating Wikipedia(Text) CC BY-SA