Where Guinevere shows heart, Ada too (6)
I believe the answer is:
nevada
'where guinevere' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'heart ada too' is the wordplay.
'heart' becomes 'nev' (I can't explain this - if you can you should give a lot more credence to this answer).
'too' is a charade indicator (letters next to each other).
'nev'+'ada'='NEVADA'
'shows' acts as a link.
Can you help me to learn more?
(Other definitions for nevada that I've seen before include "State of western USA" , "South-west US state" , "Curiously, Dan and Eva are in the US" , "Southwestern US state" , "US state, capital Carson City" .)