My lie is found out by a girl (5)
I believe the answer is:
emily
'a girl' is the definition.
'my lie is found out' is the wordplay.
'is found out' indicates anagramming the letters (discovered out of the normal letter order).
'my'+'lie'='mylie'
'mylie' anagrammed gives 'EMILY'.
'by' acts as a link.
(Other definitions for emily that I've seen before include "Girl's name" , "Woman" , "-- Dickinson, US poet" , "A Bronte sister" , "Girl" .)