Honesty in a letter to a girl (5)
I believe the answer is:
truth
'honesty' is the definition.
(truth is a kind of honesty)
'a letter to a girl' is the wordplay.
'a letter' becomes 't'.
'to a girl' becomes 'ruth' (**).
't'+'ruth'='TRUTH'
'in' acts as a link.
(Other definitions for truth that I've seen before include "Honesty" , "Belief accepted as correct" , "Candour" , "Actuality" , "Verity" .)