Top expert on the water? (7)
I believe the answer is:
surface
'top' is the definition.
(thesaurus)
'expert on the water?' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'on' means one lot of letters go next to another.
'the water?' becomes 'surf' (I can't explain this - if you can you should give a lot more credence to this answer).
'ace' after 'surf' is 'SURFACE'.
Can you help me to learn more?
(Other definitions for surface that I've seen before include "Plane" , "Outer or upper layer" , "Wake up" , "Outer boundary" , "Exterior" .)