Lady in truth (6)
I believe the answer is:
verity
This clue is a double definition.
'lady' is the first definition.
(I have seen 'A lady' mean 'verity' so perhaps 'lady' could also mean 'verity')
'in truth' is the second definition.
(Other definitions for verity that I've seen before include "Truthfulness" , "A lady" , "Truth - girl's name" .)