In Latin 'I' is used by many showing selfishness (6)
I believe the answer is:
egoism
'selfishness' is the definition.
(I've seen this before)
'in latin i is used by many' is the wordplay.
I cannot quite understand how this works, but
'i' could be 'ego' ('I' in Latin) and 'ego' is found within 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 explanation may well be incorrect...
'showing' is the link.
Can you help me to learn more?
(Other definitions for egoism that I've seen before include "Self-seeking" , "Self interest" , "Self-centredness, vanity" , "display of selfishness" , "Self-centredness, conceit" .)