Brad observed in Gretna illegally (4)

I believe the answer is:

nail

'brad' is the definition.
(brad is a nail with rectangular cross-section)

'observed in gretna illegally' is the wordplay.
'observed in' indicates a hidden word.
'NAIL' can be found hidden inside 'gretna illegally'.

(Other definitions for nail that I've seen before include "Hard plate at end of digit" , "Part of finger" , "Hit this on the head to be absolutely right" , "Fixing spike" , "it needs hammering home" .)

I've seen this clue in The Sun.

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