Is there a word for "truncate to size zero", preferably one which could be used and understood in the following way:
"…ing the file" := "truncating the file to zero bytes"?
RECAP: The best answers so far seem to be "empty the file", "zero the file size" and "truncate to (size|length) zero". In a suitable context, "truncate" has "truncate to (size|length) zero" as the default meaning (esp. since it's the commonest specific length to which files are truncated) but it's by no means the only one
Best Answer
empty the file, could be used. After all, one is left with an empty file.
There is Google evidence of usage for empty the file.
There is also Google evidence of usage for clear the file.