Any of five letters read out is a confession (6)
I believe the answer is:
avowal
'confession' is the definition.
(I've seen this in another clue)
'any of five letters read out is a' is the wordplay.
I cannot really understand how this works, but
'of' could be 'o' and 'o' is found in the answer.
'five' could be 'v' (Roman numeral) and 'v' is found within the answer.
'a' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for avowal that I've seen before include "Affirmed declaration" , "Open assertion" , "testimony?" , "Affirmation" , "Profession of" .)