Catch slowcoach heading off (4)

I believe the answer is:

nail

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'catch' is the definition.
(nail can mean to catch or arrest)

I can't explain the rest of the clue.

(Other definitions for nail that I've seen before include "Collar -- cop" , "Thin pointed fastener" , "Metal spike with head" , "Apprehend" , "Brad or tack" .)

I've seen this clue in The Telegraph.

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