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 "production of Fletcher?", "This points", "Archery projectile", "Cupid's projectile", "Missile of the archer", "It caught Harold's eye", "Missile with a point", "It may indicate", "Pointed missile" and "An archer's missile".)