Here is a quick way to keep the last 5000 lines of a large file instead of entirely clearing it.  A little bit of AWK though a simple array and bob's your uncle:

root [XYZ01] /var/spool/mail: ls -altri test.txt daemon
 1067 -rw-rw—-    1 daemon   mail       51130586 Jan 31 07:30 daemon
 1061 -rw-rw—-    1 daemon   mail         186022 Feb  1 00:20 test.txt
root [XYZ01] /var/spool/mail: cp -ip daemon test.txt
overwrite test.txt? y
root [XYZ01] /var/spool/mail: ls -altri test.txt daemon
 1061 -rw-rw—-    1 daemon   mail       51130586 Jan 31 07:30 test.txt
 1067 -rw-rw—-    1 daemon   mail       51130586 Jan 31 07:30 daemon
root [XYZ01] /var/spool/mail: FILEN="test.txt"; tail -n 5000 $FILEN|awk 'BEGIN { FILEN="'"$FILEN"'"; } { ARY[CNT++]=””$0; } END { for ( KEY in ARY ) { print ARY[KEY] > FILEN; } }'
root [XYZ01] /var/spool/mail: ls -altri test.txt daemon
 1067 -rw-rw—-    1 daemon   mail       51130586 Jan 31 07:30 daemon
 1061 -rw-rw—-    1 daemon   mail         186022 Feb  1 00:21 test.txt
root [XYZ01] /var/spool/mail: wc -l test.txt
    5000 test.txt
root [XYZ01] /var/spool/mail:
root [XYZ01] /var/spool/mail:
 

Cheers,
Tom