Extremely with-it, in truth (6)
I believe the answer is:
verity
'in truth' is the definition.
'extremely with-it' is the wordplay.
'extremely' becomes 'very' (I've seen this before).
'with' is an insertion indicator.
'very' placed around 'it' is 'VERITY'.
(Other definitions for verity that I've seen before include "A lady" , "Truth - girl's name" , "Truthfulness" .)