A right horror (5)

I believe the answer is:

alien

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

'horror' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

'a right' is the wordplay.
'right' becomes 'lien' (legal right).
'a'+'lien'='ALIEN'

Can you help me to learn more?

(Other definitions for alien that I've seen before include "Foreigner, outsider" , "Offensive" , "Foreign; extraterrestrial" , "repugnant" , "Otherworldly" .)

I've seen this clue in The Independent.

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