A smasher, once apt to lead seamen astray (7)
I believe the answer is:
wrecker
'a smasher' is the definition.
The definition and answer can be both people as well as being singular nouns.
Maybe there's an association between them I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for wrecker that I've seen before include "Person on demolition work - destroyer" , "Lurer of ships onto rocks" , "US breakdown vehicle" , "Saboteur" , "Vandal" .)