In Latin 'I' is used by many showing vanity (6)
I believe the answer is:
egoism
'vanity' is the definition.
The definition and answer can be both human attributes as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'in latin i is used by many' is the wordplay.
I cannot really see how this works, but
'i' could be 'ego' ('I' in Latin) and 'ego' is found in the answer.
'is' is within the answer.
'many' could be 'm' (Roman numeral for a thousand) and 'm' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'showing' is the link.
Can you help me to learn more?
(Other definitions for egoism that I've seen before include "display of selfishness" , "Self-centredness, vanity" , "Excessive self-regard" , "Self-centredness, conceit" , "Self-seeking" .)