Some medieval ideas are well-founded (5)
I believe the answer is:
'well-founded' is the definition.
(I've seen this in another clue)
'some medieval ideas are' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'VALID' is hidden within 'medieval ideas are'.
(Other definitions for valid that I've seen before include "Legally acceptable, unexpired" , "Still legally acceptable" , "Based on reason or law" , "Sound, well-based" , "Officially acceptable" .)