Sincere, perfect and altruistic person (6)
I believe the answer is:
honest
'sincere' is the definition.
(similar in meaning)
'perfect and altruistic person' is the wordplay.
'perfect' becomes 'hone' (synonyms).
'and' means one lot of letters go next to another.
'altruistic person' becomes 'st' (I can't justify this - if you can you should believe this answer much more).
'hone'+'st'='HONEST'
Can you help me to learn more?
(Other definitions for honest that I've seen before include "Free of deceit, truthful by nature" , "Truthful and dependable" , "Not lying" , "Not disposed to cheat" , "Honorable and sincere" .)