Brown fellow departing straight from the circle (7)
I believe the answer is:
tangent
'departing straight from the circle' is the definition.
I can't tell whether this definition defines the answer.
'brown fellow' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'fellow' becomes 'gent' (synonyms).
'tan'+'gent'='TANGENT'
Can you help me to learn more?
(Other definitions for tangent that I've seen before include "Straight line just touching a curve" , "Straight line that touches but does not intersect a circle" , "Straight line that touches but does not intersect a curve" , "At which one can go off" , "Line grazing another" .)