In truth, so full of it (6)
I believe the answer is:
verity
'in truth' is the definition.
'so full of it' is the wordplay.
'so' becomes 'very' (I've seen this before).
'full of' indicates putting letters inside.
'very' going around 'it' is 'VERITY'.
(Other definitions for verity that I've seen before include "A lady" , "Truth - girl's name" , "Truthfulness" .)