Very honest about line unknown (6)
I believe the answer is:
really
'very' is the definition.
(I've seen this before)
'honest about line unknown' is the wordplay.
'honest' becomes 'real'.
'about' is an insertion indicator.
'line' becomes 'l'.
'unknown' becomes 'y' (algebra).
'real' placed around 'l' is 'reall'.
'reall'+'y'='REALLY'
(Other definitions for really that I've seen before include "Do you mean it?" , "surely not?" , "Indeed" , "In truth - join forces again" , "In truth - one may be surprised to say it" .)