Girl I went out with proved true (9)
I believe the answer is:
validated
'proved true' is the definition.
(I've seen this before)
'girl i went out' is the wordplay.
'girl' becomes 'Val' (short for Valerie).
'went out' becomes 'dated' (dating is a kind of going out).
'val'+'i'+'dated'='VALIDATED'
'with' acts as a link.
(Other definitions for validated that I've seen before include "proved true" , "backed up" , "certified" .)