Undermine Conservative pledge to maintain order (10)
I believe the answer is:
compromise
'undermine' is the definition.
'conservative pledge to maintain order' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'pledge' becomes 'promise' (I've seen this in another clue).
'to maintain' is an insertion indicator (maintain can mean to keep or preserve).
'order' becomes 'om' (Order of Merit).
'c'+'promise'='cpromise'
'cpromise' enclosing 'om' is 'COMPROMISE'.
(Other definitions for compromise that I've seen before include "Agree a middle way" , "Give-and-take" , "Settlement" , "accommodation" , "Middle course" .)