Extremely grasping relations in truth (6)
I believe the answer is:
verity
'in truth' is the definition.
'extremely grasping relations' is the wordplay.
'extremely' becomes 'very' (very can mean extremely or considerably).
'grasping' means one lot of letters goes inside another (inserted letters are held or grasped).
'relations' becomes 'it' (both can mean sex).
'very' going around 'it' is 'VERITY'.
(Other definitions for verity that I've seen before include "Truthfulness" , "A lady" , "Truth - girl's name" .)