Back company in engaging American relative (6,6)
I believe the answer is:
second cousin
'relative' is the definition.
(second cousin is a kind of relative)
'back company in engaging american' is the wordplay.
'back' becomes 'second' (both can mean to support an idea or proposal).
'company' becomes 'co' (abbreviation).
'engaging' indicates putting letters inside.
'american' becomes 'us' (abbreviation for United States).
'co'+'in'='coin'
'coin' enclosing 'us' is 'cousin'.
'second'+'cousin'='SECOND COUSIN'
(Other definitions for second cousin that I've seen before include "Fairly distant relation" , "Relative" , "Some family" .)