Relative's back home keeping company with us (6,6)
I believe the answer is:
second cousin
'relative's' is the definition.
(second cousin is a kind of relative)
'back home keeping company with us' is the wordplay.
'back' becomes 'second' (both can mean to support an idea or proposal).
'home keeping company' becomes 'coin' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' means one lot of letters goes inside another.
'coin' going around 'us' is 'cousin'.
'second'+'cousin'='SECOND COUSIN'
Can you help me to learn more?
(Other definitions for second cousin that I've seen before include "Some family" , "Relative" , "Fairly distant relation" .)