Excessive response ended battle involving Royal Engineers (12)
I believe the answer is:
overreaction
'excessive response' is the definition.
(I've seen this before)
'ended battle involving royal engineers' is the wordplay.
'ended' becomes 'over' (synonyms).
'battle' becomes 'action' (action can mean action in war).
'involving' indicates putting letters inside.
'royal engineers' becomes 'RE' (abbreviation).
'over'+'action'='overaction'
'overaction' placed around 're' is 'OVERREACTION'.
(Other definitions for overreaction that I've seen before include "Extreme response" , "Excessive response" .)