Unfortunately , WP doesn't allow us to remove App Cache globally. It is totally dependent on Dev to add such functionality for his app (6Tag has it).
The only possible way to delete the cache is to delete the app and install it again. I do this almost every 3rd week as my 9gag app keep building large amounts of cache data , which although doesn't slow the app much but I just don't like it
A trick I found to clear WhatsApp cache (partially) is to clear chat history of groups (which usually go 250+ chats / day). This clears around 10-20 mb for me.