Top water-sport expert? (7)
I believe the answer is:
surface
'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).
'surf'+'ace'='SURFACE'
(Other definitions for surface that I've seen before include "get out of bed" , "Facade" , "Plane" , "Outer boundary" , "Upper limit of liquid" .)