Revealing source, legendary Swiss thanks the French (8)
I believe the answer is:
telltale
'revealing source' is the definition.
'telltale' can be an answer for 'revealing' (I've seen this before). I'm unsure of the 'source' bit.
'legendary swiss thanks the french' is the wordplay.
'legendary swiss' becomes 'tell' (I've seen this in another clue).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'the french' becomes 'le' ('the' in French).
'tell'+'ta'+'le'='TELLTALE'
Can you help me to learn more?
(Other definitions for telltale that I've seen before include "Person who blabs, or something revealing" , "Revealing indicator or sneak" , "Blabber of secrets" , "One revealing secrets" , "Person who gossips or blabs secrets" .)