Prisoner to droop with head hidden (3)

I believe the answer is:

lag

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'prisoner' is the definition.
(slang term)

I cannot understand how the remainder of the clue works.

(Other definitions for lag that I've seen before include "Go too slow, fail to keep up" , "Convict (slang)" , "Wrap to protect from frost" , "Fall behind; insulate" , "Fall behind or cover to prevent heat loss" .)

I've seen this clue in The Times.

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