One on the board is doing this in the main? (7)
I believe the answer is:
surfing
'one on the board is doing this' is the definition.
The definition suggests a singular noun which matches the answer.
'in the main?' is the wordplay.
I cannot quite understand how this works, but
'in' is within the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for surfing that I've seen before include "Browsing the Internet" , "sport" , "Riding waves" .)