Beat chap touching line (7)
I believe the answer is:
tangent
'touching line' is the definition.
(a line touching a circle)
'beat chap' is the wordplay.
'beat' becomes 'tan' (tan can mean to beat or hit).
'chap' becomes 'gent' (both can mean a man).
'tan'+'gent'='TANGENT'
(Other definitions for tangent that I've seen before include "line in mathematics" , "Trig ratio" , "Geometric line" , "Particular line" , "Straight line just touching a curve" .)