Quash urban legend started by girl (5)
I believe the answer is:
annul
'quash' is the definition.
(synonyms)
'urban legend started by girl' is the wordplay.
'started' suggests taking the first letters.
'by' means one lot of letters go next to another.
'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" .)