Crossword Genius

Top expert on board? (7)

Ross

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" .)

I've seen this clue in The Times.
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