Bang on about retired cleric (8)
I believe the answer is:
accurate
'bang on' is the definition.
(I've seen this before)
'about retired cleric' is the wordplay.
'about' becomes 'ca' (abbreviation for 'circa').
'retired' says the letters should be written backwards.
'cleric' becomes 'curate' (I've seen this before).
'ca' in reverse letter order is 'ac'.
'ac'+'curate'='ACCURATE'
(Other definitions for accurate that I've seen before include "telling it like it is" , "Spot-on" , "Totally correct" , "Correct in all details" , "Spot on" .)