It might be oblique note to follow Mark, perhaps: ringing endorsement (6)
I believe the answer is:
stroke
'it might be oblique' is the definition.
The definition suggests a singular noun which matches the answer.
'note to follow mark perhaps ringing endorsement' is the wordplay.
'note' becomes 'e' (musical note).
'to' says to put letters next to each other (I've seen this in other clues).
'follow mark perhaps' becomes 'str' (I can't explain this - if you can you should give a lot more credence to this answer).
'ringing' is an insertion indicator.
'endorsement' becomes 'ok' (OK is an example).
'e' put after 'str' is 'stre'.
'stre' placed around 'ok' is 'STROKE'.
Can you help me to learn more?
(Other definitions for stroke that I've seen before include "Blow ?- single movement" , "Rub gently" , "A blow or brain seizure" , "Brush gently - coup" , "breast, for one" .)