Beat chap — it's not the main issue (7)
I believe the answer is:
tangent
'not the main issue' is the definition.
I can't tell whether this defines the answer.
'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'
'it's' is the link.
Can you help me to learn more?
(Other definitions for tangent that I've seen before include "mathematician's line" , "Straight line that touches but does not intersect a curve" , "Particular line" , "Completely different line of thought - trigonometrical function" , "seen to be touching" .)