Relation is mostly vague (5)
I believe the answer is:
uncle
'relation' is the definition.
(uncle is a kind of relation)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for uncle that I've seen before include "He's related" , "Family relation" , "Pawnbroker (sl.)" , "A parent's brother" , "Brother of one's parent" .)