Truth, really, about sex (6)
I believe the answer is:
verity
'truth' is the definition.
(verity is a kind of truth)
'really about sex' is the wordplay.
'really' becomes 'very' (I've seen this before).
'about' indicates putting letters inside.
'sex' becomes 'it' (a common euphemism).
'very' placed around 'it' is 'VERITY'.
(Other definitions for verity that I've seen before include "Truth - girl's name" , "A lady" , "Truthfulness" .)