Back firm briefly employing family member (6,6)
I believe the answer is:
second cousin
'family member' is the definition.
(I know that second cousin is a type of relation)
'back firm briefly employing' is the wordplay.
'back' becomes 'second' (both can mean to support an idea or proposal).
'firm' becomes 'co' (a firm is a company).
'briefly' means to remove the last letter.
'employing' becomes 'using' (to employ is to use).
'using' with its last letter taken away is 'usin'.
'second'+'co'+'usin'='SECOND COUSIN'
(Other definitions for second cousin that I've seen before include "Fairly distant relation" , "Some family" , "Relative" .)