Behold engraver's primary filling so intricately lined? (8)
I believe the answer is:
nielloed
'intricately lined?' is the definition.
The definition suggests a past participle verb which matches the answer.
'behold engraver's primary filling' is the wordplay.
I cannot quite understand how this works, but
'behold' could be 'lo' (archaic exclamation meaning 'look!') and 'lo' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'so' is the link.
Can you help me to learn more?
(Another definition for nielloed that I've seen is " engraving?".)