Top expert on waves (7)
I believe the answer is:
surface
'top' is the definition.
(thesaurus)
'expert on waves' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'on' says to put letters next to each other.
'waves' becomes 'surf' (I've seen this before).
'ace' put after 'surf' is 'SURFACE'.
(Other definitions for surface that I've seen before include "Upper limit of liquid" , "Facade" , "Superficial appearance" , "Outer or upper layer" , "Worktop" .)