Pointer to a royal line (5)
I believe the answer is:
Here is my best explanation:
'pointer' is the definition.
('arrow' can be a synonym of 'pointer')
'a royal line' is the subsidiary indication.
'royal' becomes 'r' (abbreviation).
'line' becomes 'row' (row is a kind of line**).
'to' is the link.
(Other definitions for arrow that I've seen before include "What's shot from a bow", "Projectile from bow", "It caught Harold's eye", "The bowman's missile", "William Tell's missile", "Pointed projectile", "Mark to indicate direction", "Missile with a point", "Weapon projectile" and "Pointer; dart".)