Hi fragumon......welcome to TFF.
Hmmmm......I've been "playing with guns" for about 60 years and I've never heard of it before.
Might it be a local or regional term?
AHA!!!! I Googled "docker's clutch" and apparently it's a rig somewhat like a shoulder holster.....
"She tested the gunbelt again. It was slung across her bosom almost like a shoulder-holster (an arrangement Roland thought of as a docker's clutch
) and looked simple enough, but it had taken many weeks of trial and error -- and a great deal of tailoring -- to get it just right. The belt and the revolver which cocked its eroded sandalwood grip out of the ancient oiled holster had once been the gunslinger's; the holster had hung on his right hip. He had spent much of the last five weeks coming to realize it was never going to hang there again. Thanks to the lobstrosities, he was strictly a lefthanded gun now."