I doubt your clutch is failed. The more prevalent failure mode is to have the fan fail to turn with the engine even when the engine is hot enough to have activated the clutch, not lock up. If you check the car first thing in the morning after it has had a chance to cool you should get more representative results. Start the car, let it run for a minute or so, then shut it off and see if the fan blades turn by hand.
In reality the fan blades should be hard to turn when the clutch is engaged, but not impossible. It is a fluid clutch (unless your vehicle has an electromagnetic one, in which case I am not sure of its behaviour with the clutch engaged and the motor not running), and the design limits the torque the clutch can transfer, which limits the speed of the blade wheel. So, when engaged it runs like it was locked up until the engine rpm gets over about 2500 or so, when it begins to slip a bit. By 3,000 rpm the clutch reaches its maximum torque transmission limit and the fan will not speed up when the engine speed increases.
I hope this helps. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)