They're smashing - if you're a good swimmer! (8)
I believe the answer is:
breakers
'swimmer' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for breakers that I've seen before include "Large waves beloved of surfers" , "Large waves or wreckers" , "White-topped waves" , "Smashing types" , "Rollers" .)