Corps joining battle in response (8)
I believe the answer is:
reaction
'response' is the definition.
(I know that response can be written as reaction)
'corps joining battle' is the wordplay.
'corps' becomes 'RE' (Royal Engineers army regiment).
'joining' means one lot of letters go next to another.
'battle' becomes 'action' (military action).
're'+'action'='REACTION'
'in' acts as a link.
(Other definitions for reaction that I've seen before include "counter-revolution" , "Chemical change or backlash" , "Force" , "Chemical change or response" , "Result of stimulus" .)