Set null on field after submit data on edit page

What to set a nulll in camp after submit the data on edit

I need to get information posted and send it on e-mail on edit page and after set null to this field

it is possible?
My tentative no succes i have this on BeforeEdit event but no succes

I have "Send email with new data" to send necessary data

//** Send email with new data ****

$email="diego@sysmo.com.br";

$message="";

$subject="Nova confirmação de treinamento";
foreach($values as $field=>$value)

$message.= $field." : ".$value."\r\n";
mail($email, $subject, $message);
global $conn;

$strSQLExists = "update cli_treinamento set obs_cli = null where id = $idx";

$rsExists = db_query($strSQLExists,$conn);