Top expert on board? (7)
I believe the answer is:
surface
'board?' is the definition.
(board is a kind of surface)
'top expert on' is the wordplay.
'top' becomes 'surf' (I can't explain this - if you can you should give a lot more credence to this answer).
'expert' becomes 'ace' (ace can informally mean very good).
'on' means one lot of letters go next to another.
'surf'+'ace'='SURFACE'
(Other definitions for surface that I've seen before include "Exterior" , "Plane" , "In water, come to the top" , "Outside part" , "Become apparent" .)