Separate publisher engaged by one of the family (8)
I believe the answer is:
uncouple
'separate' is the definition.
(I've seen this in another clue)
'publisher engaged by one of the family' is the wordplay.
'publisher' becomes 'OUP' (abbreviation for Oxford University Press).
'engaged by' means one lot of letters goes inside another (I've seen this in other clues).
'one of the family' becomes 'uncle'.
'oup' going inside 'uncle' is 'UNCOUPLE'.
(Other definitions for uncouple that I've seen before include "Separate (carriage from train)" , "Detach one train carriage from another" , "Disconnect" , "Detach (a railway carriage)" , "Release" .)