One sharing with me has opportunity to finish board game (8)
I believe the answer is:
roommate
'one sharing with me has opportunity to finish' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'board game' is the wordplay.
'board' becomes 'room' ('room' can be a synonym of 'board').
'game' becomes 'mate' (I can't justify this - if you can you should give a lot more credence to this answer).
'room'+'mate'='ROOMMATE'
Can you help me to learn more?
(Other definitions for roommate that I've seen before include "One sharing accommodation" , "Person with whom one shares" , "Fellow lodger" , "Accommodation sharer" , "Stable companion" .)