Quash urban legend started by girl (5)
I believe the answer is:
annul
'quash' is the definition.
(I know that quash can be written as annul)
'urban legend started by girl' is the wordplay.
'started' suggests taking the first letters.
'by' says to put letters next to each other.
'girl' becomes 'ann' (woman's name).
The first letters of 'urban legend' is 'ul'.
'ul' after 'ann' is 'ANNUL'.
(Other definitions for annul that I've seen before include "Obligation" , "Declare void" , "Nullify" , "Declare (a law) invalid" , "Cancel officially" .)