You've never met this faultless unknown! (7,8)
I believe the answer is:
perfect stranger
'you've never' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'met this faultless unknown' is the wordplay.
'met' is a charade indicator (letters next to each other).
'this faultless' becomes 'perfect' (I've seen this before).
'unknown' becomes 'stranger' (synonyms).
'perfect'+'stranger' is 'PERFECT STRANGER'.
Can you help me to learn more?
(Another definition for perfect stranger that I've seen is " one one's not met before".)