Top water-sport expert? (7)
I believe the answer is:
'top' is the definition.
(I've seen this before)
'water-sport expert?' is the wordplay.
'water sport' becomes 'surf' (surfing).
'expert?' becomes 'ace' (ace can informally mean very good).
(Other definitions for surface that I've seen before include "get out of bed" , "Facade" , "Plane" , "Outer boundary" , "Upper limit of liquid" .)