That's an old wives' tale, for example, discovered in advance (6)
I believe the answer is:
legend
'that's an old wives' tale' is the definition.
I can't tell whether this defines the answer.
'example discovered in advance' is the wordplay.
'example' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'discovered in' means one lot of letters goes inside another.
'advance' becomes 'lend' (I've seen this in another clue).
'eg' inserted within 'lend' is 'LEGEND'.
'for' is the link.
Can you help me to learn more?
(Other definitions for legend that I've seen before include "Tale; caption" , "Roger Federer, for example" , "Tale of folklore" , "Celebrity" , "Story shows the opposite" .)