It's 'for Clevedon' only in the sense that Yatton is the railhead for Clevedon.
There is however a multi-mode, multi-operator ticket you could use. It's called the Freedom Travelpass, valid on trains in a defined area and most buses in said area, and comes in various flavours. For your journey if you wanted to combine train and bus it would cost ^13.50 peak and ^12.50 after 9am for a day ticket. There are also weekly and monthly seasons available.
More information:
FGW▸ :
https://www.firstgreatwestern.co.uk/your-journey/travel-connections/freedom-travel-passFirst Bus:
http://www.firstgroup.com/ukbus/bristol_bath/tickets/bus_rail/freedom.phpAs for a connecting bus, it would be better to travel to Nailsea and Backwell station, but even then you have a longish walk to Nailsea town centre to pick up First's X8 to Clevedon. The only bus that runs from Yatton station to Clevedon is Carmel Coaches' 88. With a grand total of 4 buses a day! I'm unsure though if Carmel are included in the list of operators on which the Freedom Travelpass is valid.
In summary I think you'd be better off just getting the X8 from Bristol Bus Station through to Clevedon. Day ticket ^6.