Line established by man on beat (7)
I believe the answer is:
tangent
'line' is the definition.
(I know that tangent is a type of straight line)
'man on beat' is the wordplay.
'man' becomes 'gent' (gent is a kind of man).
'on' is a charade indicator (letters next to each other).
'beat' becomes 'tan' (tan can mean to beat or hit).
'gent' put after 'tan' is 'TANGENT'.
'established by' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for tangent that I've seen before include "Straight line touching a circle" , "A new line of thought" , "Completely different line of thought - trigonometrical function" , "Straight line that touches but does not intersect a circle" , "Trig ratio" .)