Opposition from Royal Engineers is over policy (10)
I believe the answer is:
resistance
'opposition' is the definition.
('resistance' can be a synonym of 'opposition')
'royal engineers is over policy' is the wordplay.
'royal engineers' becomes 're'.
'over' says the letters should be written in reverse.
'policy' becomes 'stance' (I've seen this before).
'is' in reverse letter order is 'si'.
're'+'si'+'stance'='RESISTANCE'
'from' acts as a link.
(Other definitions for resistance that I've seen before include "Violent opposition" , "immunity" , "underground movement" , "Freedom fighters - it's measured in ohms" , "fighters in wartime" .)