Source of ink to mark sheep - arcane symbol? (9)
I believe the answer is:
pentagram
'arcane symbol?' is the definition.
(pentagram is a mystical symbol)
'source of ink to mark sheep' is the wordplay.
'source of ink' becomes 'pen'.
'to mark' becomes 'tag' ('tag' can be a synonym of 'mark').
'sheep' becomes 'ram' (ram is a kind of sheep).
'pen'+'tag'+'ram'='PENTAGRAM'
(Other definitions for pentagram that I've seen before include "Mystical symbol" , "Star-shaped figure" , "Five pointed star" , "Five-pointed star" , "magical symbol" .)