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A line in picture indicating muscle (7)

Ross

I believe the answer is:

scalene

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'a line in' is the definition.
The definition suggests an adjective which matches the answer.

I cannot understand how the rest of the clue works.

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(Other definitions for scalene that I've seen before include "(Of triangle) with each side of a different length" , "Unequal-sided (triangle)" , "Describing a triangle with unequal sides" , "Neither equilateral nor isosceles" , "(Of triangle) with no sides equal" .)

I've seen this clue in The Sunday Times and The Times.
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