Very high roller (7)
I believe the answer is:
tsunami
This might be a double definition.
'very high' is the first definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'roller' is the second definition.
(thesaurus)
Can you help me to learn more?
(Other definitions for tsunami that I've seen before include "Huge wave caused by earth tremor" , "Great, fast-moving sea wave" , "Huge destructive sea wave" , "I am nuts! (anag)" , "wall of water" .)