Undo car out of sincerity (7)
I believe the answer is:
candour
'sincerity' is the definition.
(I've seen this before)
'undo car out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'undo'+'car'='undocar'
'undocar' anagrammed gives 'CANDOUR'.
'of' acts as a link.
(Other definitions for candour that I've seen before include "Openness, honesty" , "'Frankness, honesty (7)'" , "Forthrightness" , "telling it like it is" , "open)" .)