One dividing the archaic Left deserted before Miliband returned (7)
I believe the answer is:
yielded
'returned' is the definition.
('yield' can be a synonym of 'return')
'one dividing the archaic left deserted before' is the wordplay.
'one dividing' is an insertion indicator (inserted letters divide the word).
'the archaic left' becomes 'yield' (I can't explain this - if you can you should give a lot more credence to this answer).
'deserted' becomes 'd'.
'before' becomes 'e' (I've seen this in other clues).
'd'+'e'='de'
'yield' placed around 'de' is 'YIELDED'.
'miliband' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
(Other definitions for yielded that I've seen before include "Generated (financial return)" , "Gave way under pressure" , "Capitulated" , "abandoned" , "Bore fruit" .)