Use the variable which is used by your for each loop container in a Derived Column Transformation.

Set Derived Column Name to Clientname, leave Derived Column as <add as new column> and set Expression to:

SUBSTRING (@YourVariable, 1, FINDSTRING(@YourVariable,"_",1)-1)

Add a New line for Derived Column PartnerName with expression SUBSTRING (@YourVariable, FINDSTRING(@YourVariable,"_",1)+1, FINDSTRING(@YourVariable,"_",2)-1)

Add a new line for Derived Column Entity with expression SUBSTRING (@YourVariable, FINDSTRING(@YourVariable,"_",2)+1, FINDSTRING(@YourVariable,"_",3)-1)

(For SSIS 2012 and above you can use the TOKEN function instead of the SUBSTRING/FINDSTRING combination)