A smasher, once apt to lead seamen astray (7)

I believe the answer is:

wrecker

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'a smasher' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand the remainder of the clue.

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" .)

I've seen this clue in the Evening Standard.

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