A wave from a smasher! (7)
I believe the answer is:
breaker
'a wave' is the definition.
(breaker is a kind of wave)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for breaker that I've seen before include "Roller?" , "A large wave, sounds destructive" , "Wave with white crest" , "one stopping, might you say?" , "White-topped wave" .)