Crossword Genius

Defect found after using imperial measurement - they've crossed a line making this error (9)

Ross

I believe the answer is:

footfault

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'measurement they've crossed a line making this error' is the definition.
'footfault' can be an answer for 'error' (footfault is a kind of error). I am unsure of the remainder of the definition.

'defect found after using imperial' is the wordplay.
'defect' becomes 'fault' (synonyms).
'found after' is a charade indicator (letters next to each other).
'using imperial' becomes 'foot' (I am not sure about this - if you are sure you should believe this answer much more).
'fault' put after 'foot' is 'FOOTFAULT'.

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I've seen this clue in The Irish Times.
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