lcErg = ""
for lnRun to alines(laLines, FileToStr("c:\mydir\m.txt"))
    If !Empty(laLines[m.lnRun])
       lcErg = m.lcErg + laLines[m.lnRun]
    endif
next
= StrToFile(m.lcErg, "c:\mydir\m_Clean.txt")

Overwriting with the same name might get you into other troubles,
so I prefer to keep original file unchanged.

HTH

thomas