The Tower, perhaps, where one may have lost one's head? (5)

I believe the answer is:

block

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'head?' is the definition.
(a barber's block is a head for storing a wig)

I don't understand how the remainder of the clue works.

(Other definitions for block that I've seen before include "Impede, prevent" , "Obstruct" , "Solid lump" , "cake?" , "Solid shape; prevent" .)

I've seen this clue in the Evening Standard.

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