Copulation of foxes; and thence used, in a canting sense, for that of men and women: as, The cull and the mort are at clicket in the dyke; the man and woman are copulating in the ditch.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Clicker * ClimbNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes