Outward appearance of water sport champion (7)
I believe the answer is:
surface
'outward appearance' is the definition.
(I've seen this before)
'water sport champion' is the wordplay.
'water sport' becomes 'surf' (surfing).
'champion' becomes 'ace' (both can mean a person excelling at something).
'surf'+'ace'='SURFACE'
'of' is the link.
(Other definitions for surface that I've seen before include "Exterior" , "Uppermost layer" , "Outside part" , "Worktop" , "Upper limit of liquid" .)