How do you delete one line from a [file]? What kind of file are you talking about? 
There aren't, in general, lines in [MP3] files, or [GIF] files, etc.
In fact, most operating systems today don't have operating system calls
that deal in lines. 
Instead, they come with what I have named, at times, "meta-read" functions - 
pieces of code which read in chunks of bytes from a file, and then, 
depending on the function's intention, add some code 
implementing a common interpretation of some of those bytes.

Most people, asking this particular question, are ''really'' asking
"Assume that I have a plain, traditional, text file - containing 7 bit [ASCII] 
characters, where newline character indicate the end of a "line". 
How can I delete one of those "lines" ."

And honestly, I really believe that's what is being asked. So the following
attempts to address that specific interpretation of this question.

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Here's an example:

    set tmpname /tmp/something

    set source [open $filename]
    set destination [open $tmpname w]
    set content [read $source]
    close $source
    set lines [split $content \n]
    set lines_after_deletion \
       [lreplace $lines $line_number_to_remove $line_number_to_remove]
    puts -nonewline $destination [join $lines_after_deletion \n]
    close $destination
    file rename -force $destination $source


Note, however, that in both of these cases, what you are doing 
are reading the entire file and writing out the entire file, counting lines.
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[RS]: Here is a case where [awk] is simpler than if we do it in [Tcl]. 
Say, you want all but the 4th line:
 gawk 'NR!=4' infile > outfile

[sed] is also a candidate:

  sed -e -n "4!p" infile >outfile

[[anyone know a more concise ('''-n'''-less?) way to achieve the same?]]
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[Category Example] - [Category File]