Replace start of session with short online forum? That's a revelation! (10)
I believe the answer is:
confession
'that's a revelation' is the definition.
(thesaurus)
'replace start of session with short online forum?' is the wordplay.
I cannot quite understand how this works, but
'of' could be 'o' and 'o' is located in the answer.
'online' could be 'on' (similar in meaning) and 'on' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for confession that I've seen before include "Acknowledgement of sins" , "Admission" , "revelation?" .)