Crossword Genius

Gnat swerves back before net is cast on different course (7)

Ross

I believe the answer is:

tangent

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But I can't explain it at all! Could you help me understand?

(Other definitions for tangent that I've seen before include "Straight line that touches but does not intersect a curve" , "line in mathematics" , "Line grazing another" , "Particular line" , "mostly irrelevant item" .)

I've seen this clue in The Mirror.
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