Conservative with up-to-date reforms -- this could change the government (4,5)
I believe the answer is:
coup detat
'this could change the government' is the definition.
(a coup d'etat overthrows a government)
'conservative with up-to-date reforms' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'with' indicates putting letters inside.
'reforms' indicates anagramming the letters.
'to'+'date'='todate'
'todate' with letters rearranged gives 'odetat'.
'up' inserted within 'odetat' is 'oupdetat'.
'c'+'oupdetat'='COUP DETAT'
(Other definitions for coup detat that I've seen before include "rebellion" , "revolutionary action" , "Seizure of the levers of power" , "Hostile takeover" , "leadership change" .)