To generate the patch for the given code slice, we need to follow a structured approach to identify the root cause of the vulnerability and then apply an appropriate fix. Here are the detailed reasoning steps:

### Step 1: Root Cause Analysis

1. **Memory Allocation:**
   - At line 4, memory is allocated for `clear` using `calloc`.
   - `calloc` initializes the allocated memory to zero and returns a pointer to the allocated memory.

2. **Conditional Check:**
   - At line 5, there is a check to see if the memory allocation was successful (`if (clear)`).

3. **Nested Memory Allocation:**
   - At line 8, another memory allocation is performed by calling `nsc_context_new()`, which presumably allocates memory for `clear->nsc`.

4. **Failure Handling:**
   - At line 9, there is a check to see if the allocation for `clear->nsc` was successful (`if (!clear->nsc)`).
   - If the allocation fails, the function returns `NULL` at line 11.

### Identifying the Vulnerability

- **Memory Leak:**
  - If the allocation for `clear->nsc` fails, the function returns `NULL` without freeing the previously allocated memory for `clear`.
  - This results in a memory leak because the memory allocated at line 4 is not freed before returning.

### Step 2: Fixing Strategy

To fix the memory leak, we need to ensure that any allocated memory is properly freed before returning from the function in case of an error. Specifically, if the allocation for `clear->nsc` fails, we should free the memory allocated for `clear` before returning `NULL`.

### Applying the Fix

The fix involves adding a call to `free(clear)` before returning `NULL` at line 11. Here is the modified code with the patch applied:

```diff
<         {
<             return NULL;
---
>         {
>             free(clear);
>             return NULL;
```

### Conclusion

By adding the `free(clear);` statement before returning `NULL`, we ensure that the memory allocated for `clear` is properly freed in case of an error, thus preventing a memory leak. This addresses the CWE-401 (Memory Leak) vulnerability in the code.

### Final Patched Code

Here is the final patched version of the code:

```c
1 CLEAR_CONTEXT *clear_context_new(BOOL Compressor)
2 {
3     CLEAR_CONTEXT *clear;
4     clear = (CLEAR_CONTEXT *)calloc(1, sizeof(CLEAR_CONTEXT));
5     if (clear)
6     {
7         clear->Compressor = Compressor;
8         clear->nsc = nsc_context_new();
9         if (!clear->nsc)
10         {
11             free(clear);
12             return NULL;
13         }
14         nsc_context_set_pixel_format(clear->nsc, RDP_PIXEL_FORMAT_R8G8B8);
15         clear->TempSize = 512 * 512 * 4;
16         clear->TempBuffer = (BYTE *)malloc(clear->TempSize);
17         clear_context_reset(clear);
18     }
19     return clear;
20 }
```

This ensures that the memory allocated for `clear` is properly freed if the allocation for `clear->nsc` fails, thus preventing a memory leak.