A line in picture indicating muscle (7)
I believe the answer is:
scalene
'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.
Can you help me to learn more?
(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" .)