As far back as the early 1990's the phraseology published in JSP318A was:
ATC: "C/S Clear Land" AC: "Clear Land C/S", differing slightly from the equivalent in CAP 413 "Cleared to...".
It certainly was (and to a lesser extent still is) an incorrect but common practice amongst UK military pilots to drop the "Clear". I don't know whether this was ever the published phraseology but, as BEagle says, the error was propagated by CFS.