Surviving letter to the Corinthians that’s unknown within New Testament (6)
I believe the answer is:
extant
'surviving' is the definition.
(synonyms)
'letter to the corinthians that's unknown within new testament' is the wordplay.
'letter to the corinthians' becomes 'eta' (I can't justify this - if you can you should give a lot more credence to this answer).
'that's unknown' becomes 'x' (algebra).
'within' indicates putting letters inside.
'new testament' becomes 'nt'.
'eta' enclosing 'x' is 'exta'.
'exta'+'nt'='EXTANT'
Can you help me to learn more?
(Other definitions for extant that I've seen before include "Still existing or current" , "Still in existence, surviving" , "left" , "Still existing, actual" , "Next at being still existing" .)