Line in brown followed by fellow (7)
I believe the answer is:
tangent
'line' is the definition.
(tangent is a kind of line)
'brown followed by fellow' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'followed by' is a charade indicator (letters next to each other).
'fellow' becomes 'gent' ('gent' can be a synonym of 'fellow').
'tan'+'gent'='TANGENT'
'in' acts as a link.
(Other definitions for tangent that I've seen before include "Line touching curve at one point" , "A new line of thought" , "Straight line that touches but does not intersect a curve" , "seen to be touching" , "mathematician's line" .)