IFERROR, INDEX, MATCH returning zeros instead of blanks

What sort of values is your formula returning? If they are text values it's sufficient to concatenate a "null string" to your INDEX/MATCH formula like this:

=IFERROR(INDEX('Cleaned Post'!W:W,MATCH(Combined!$C2,'Cleaned Post'!$C:$C,0))&"","")

That also works for numbers except it will convert them to text so if you don't want that you can try this version:

=IFERROR(IF(INDEX('Cleaned Post'!W:W,MATCH(Combined!$C2,'Cleaned Post'!$C:$C,0))="","",INDEX('Cleaned Post'!W:W,MATCH(Combined!$C2,'Cleaned Post'!$C:$C,0))),"")