Truth the ne plus ultra really involves (6)
I believe the answer is:
verity
'truth' is the definition.
(synonyms)
'ne plus ultra really involves' is the wordplay.
'ne plus ultra' becomes 'it' (I am not sure about this - if you are sure you should believe this answer much more).
'really' becomes 'very' (I've seen this in another clue).
'involves' indicates putting letters inside.
'it' put inside 'very' is 'VERITY'.
'the' is the link.
Can you help me to learn more?
(Other definitions for verity that I've seen before include "Truthfulness" , "A lady" , "Truth - girl's name" .)