Genuine error — Queen held in church (7)
I believe the answer is:
sincere
'genuine' is the definition.
('sincere' can be a synonym of 'genuine')
'error queen held in church' is the wordplay.
'error' becomes 'sin' (I've seen this in other clues).
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'held in' is an insertion indicator.
'church' becomes 'ce' (abbreviation for Church of England).
'er' going within 'ce' is 'cere'.
'sin'+'cere'='SINCERE'
(Other definitions for sincere that I've seen before include "Honest, heartfelt" , "Genuine and honest" , "Free from pretence" , "like informal correspondents?" , "'Genuine, not deceitful (7)'" .)