Reduction, as included in order (8)
I believe the answer is:
decrease
'reduction' is the definition.
('decrease' can be a synonym of 'reduction')
'as included in order' is the wordplay.
'included in' means one lot of letters goes inside another.
'order' becomes 'decree' ('decree' can be a synonym of 'order').
'as' inserted into 'decree' is 'DECREASE'.
(Other definitions for decrease that I've seen before include "Dock" , "Plummet" , "Turn down" , "Grow smaller or less" , "subsidence" .)