Wrong but prevalent notion (7)
I believe the answer is:
fallacy
'wrong but prevalent notion' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for fallacy that I've seen before include "Illogical argument" , "Faulty argument" , "Incorrect notion based on false reasoning" , "'Misconception, false belief (7)'" , "Wrong but prevalent notion" .)