With regard to something to be chewed over, editor concentrated (7)
I believe the answer is:
Here is my best explanation:
'concentrated' is the definition.
('reduce' can be a synonym of 'concentrate')
'with regard to something to be chewed over editor' is the subsidiary indication.
'with regard' becomes 're'.
'to something to be chewed' becomes 'cud' (I've seen this before).
'over' shows that the letters should be reversed in order.
'editor' becomes 'ed'.
'cud' back-to-front is 'duc'.
(Other definitions for reduced that I've seen before include "Got rid of some bodily weight", "Lowered in price", "Lessened", "Impoverished", "Abridged", "looked smaller", "Curtailed", "Discounted", "Made smaller" and "Decreased, cut down".)