Letter to a girl, in fact (5)
I believe the answer is:
truth
'in fact' is the definition.
(thesaurus)
'letter to a girl' is the wordplay.
'letter' becomes 't'.
'to a girl' becomes 'ruth' (girl's name).
't'+'ruth'='TRUTH'
(Other definitions for truth that I've seen before include "Honesty" , "Belief accepted as correct" , "known facts" , "Candour" , "Verity" .)