Top expert on breakers (7)
I believe the answer is:
surface
'top' is the definition.
(thesaurus)
'expert on breakers' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'on' is a charade indicator (letters next to each other).
'breakers' becomes 'surf' ('surf' can be a synonym of 'breakers').
'ace' after 'surf' is 'SURFACE'.
(Other definitions for surface that I've seen before include "Outside; rise to the top" , "Become apparent" , "Superficial appearance" , "Outside part" , "Top spot" .)