The photograph is of a group of about forty, mostly young men in a variety of military uniforms, arranged in the middle of a tented camp, with a backdrop of large trees. The word, "CLUMBER," and number, "606.," were obviously inked in pen on the negative, as they appear in white at the lower right and lower left of the front of the postcard, respectively.
The photograph has a standard divided back postcard format, with the photographer's name printed on the reverse, "Photo by H.P. Hansen, Ashbourne." H.P. Hansen operated a studio in Ashbourne, Derbyshire from the late 1890s until at least 1922. He travelled fairly widely to produce general views of popular Derbyshire attractions, as well for commissions such as group portraits tailored for particular clients. This appears to be an example of the latter.
I'm not particularly strong on uniforms, but I am aware that the regiment commonly known as the Sherwood Foresters drew heavily for its ranks from the young men of Derbyshire and Nottinghamshire. A simple Google search using the terms "Clumber" and "Sherwood Foresters" hit the jackpot!
"C Coy", 6th Battalion Sherwood Forester at Clumber Park, 1913
Image courtesy of Mike Briggs Chesterfield Sherwoods on the Somme
On Mike Briggs' excellent tribute to the men of Chesterfield (Derbyshire) who served with the Sherwood Foresters during the Battle of the Somme (Chesterfield Sherwoods on the Somme) he includes two photographs of 6th Battalion, "C" Company (Ashbourne and Buxton), Sherwood Forester Regiment at Clumber Park in 1913. He explains the formation of the unit prior to the Great War as follows:
Following the 1909 reorganisation of the Derbyshire Volunteers into the Territorial Force, the 6th Battalion Sherwood Foresters comprised the following eight companies :-
"A" - Chesterfield
"B" - Chapel-en-le-Frith
"C" - Ashbourne and Buxton
"D" - Bakewell, Tideswell and Stoney Middleton
"E" - Wirksworth and Matlock
"F" - Staveley and District
"G" - Clay Cross and District
"H" - Whaley Bridge, New Mills and Hayfield
An enlarged view of this more formal group portrait shows a very similar range of uniforms and tents. There are plenty of trees around, which is understandable if the location is indeed Clumber. According to the web site of the National Trust, which owns it, Clumber Park is near Worksop in Nottinghamshire and comprises "peaceful woodlands, open heath and rolling farmland, with a superb serpentine lake at its heart and the longest avenue of lime trees in Europe." It was a country park partly designed by Capability Brown, and owned by the Dukes of Newcastle. This image also has a number, "637," suggesting that was also by Hansen, from the same sequence, and possibly taken on the same occasion.