Response from sappers joining battle (8)
I believe the answer is:
reaction
'response' is the definition.
(I know that response can be written as reaction)
'sappers joining battle' is the wordplay.
'sappers' becomes 'RE' (nickname for the Royal Engineers).
'joining' means one lot of letters go next to another.
'battle' becomes 'action' (military action).
're'+'action'='REACTION'
'from' acts as a link.
(Other definitions for reaction that I've seen before include "effect of drugs?" , "Into care (anag)" , "Feedback" , "In mechanics, every action has an equal and opposite one" , "Chemical change or response" .)