The very thing is left out (6)
I believe the answer is:
itself
'the very thing' is the definition.
(I've seen this before)
'is left out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'is'+'left'='isleft'
'isleft' is an anagram of 'ITSELF'.
(Other definitions for itself that I've seen before include "Pronoun used to reflect the object or for emphasis" , "alone by this" , "Emphatic pronoun for the thing or animal under discussion" , "Reflexive form of an inanimate third person pronoun" , "Reflexive word - stifle (anag)" .)