Large wave (7)
(Other definitions for tsunami that I've seen before include "Marine natural disaster" , ""Harbour wave"" , "Large tidal wave" , "I am nuts! (anag)" , "wall of water" .)
'large wave' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for breaker that I've seen before include "White-topped wave" , "from rider disciplining horse?" , "Wave hitting the shore" , "Heavy wave arriving on the shore" , "A large wave, sounds destructive" .)