Locks perhaps not totally secure? (6)

I believe the answer is:

toupee

If this is the answer I cannot explain how the clue works.

(Other definitions for toupee that I've seen before include "An American wig or hair-piece" , "A wig for American male" , "An American man's wig" , "False hairpiece" , "A man's hairpiece in America" .)

I've seen this clue in The Times.

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