Crossword Genius

Bad European function (4)

Ross

I believe the answer is:

sine

'function' is the definition.
(mathematical function)

'bad european' is the wordplay.
'bad' becomes 'sin' ().
'european' becomes 'E' (abbreviation e.g. EU).
'sin'+'e'='SINE'

(Other definitions for sine that I've seen before include "Indefinitely, with no appointed date - . . . . die" , "Trig function" , "Mathematical function" , "Opposite/hypoteneuse" , "Without; maths function" .)

I've seen this clue in The Telegraph.
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