The function shifts the first character of the input string by 18 positions in the ASCII table and keeps the remaining characters unchanged.

[CODE]:
```python
def f_approx(x):
    first_char = chr(ord(x[0]) + 18)
    return first_char + x[1:]
```
Please note that this approximation assumes the input string has at least one character and might behave differently for non-letter characters as the first character. The actual function implementation might include additional constraints or behaviors.