Overrule order in stopping assignation (8)
I believe the answer is:
dominate
'overrule' is the definition.
'order in stopping assignation' is the wordplay.
'order' becomes 'om' (order of merit).
'stopping' is an insertion indicator (stopping can mean blocking a gap).
'assignation' becomes 'date' (I've seen this before).
'om'+'in'='omin'
'omin' placed inside 'date' is 'DOMINATE'.
(Other definitions for dominate that I've seen before include "Rule" , "Govern" , "Tower over" , "Command, control" , "corner" .)