Helix not designed with a piece of rock from somewhere else (8)

I believe the answer is:

xenolith

I'm a little stuck... Click here to teach me more about this clue!

'a piece of rock from somewhere else' is the definition.
I can't tell whether this defines the answer.

'helix not designed' is the wordplay.
'designed' indicates anagramming the letters (designed with the letters).
'helix'+'not'='helixnot'
'helixnot' anagrammed gives 'XENOLITH'.

'with' acts as a link.

Can you help me to learn more?

I've seen this clue in the Irish Times.

Want a hint initially instead of a full solution? Install my app