Change one's mind over government's last spending plan (6)
I believe the answer is:
budget
'spending plan' is the definition.
(I've seen this in another clue)
'change one's mind over government's last' is the wordplay.
'change one's mind' becomes 'budge' (budge from your original position).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'last' says to take the final letters.
The final letter of 'government' is 't'.
'budge'+'t'='BUDGET'
(Other definitions for budget that I've seen before include "Chancellor of the Exchequer's responsibility?" , "Plan financially" , "spending money" , "Plan for expenditure and fund allocation" , "Plan of income and expenditure" .)