Conservatism of troops before battle (8)
I believe the answer is:
reaction
'conservatism' is the definition.
(reaction is a kind of conservatism)
'troops before battle' is the wordplay.
'troops' becomes 'RE' (Royal Engineers army regiment).
'before' is a charade indicator (letters next to each other).
'battle' becomes 'action' (military action).
're'+'action'='REACTION'
'of' is the link.
(Other definitions for reaction that I've seen before include "effect of drugs?" , "Reflex; backlash" , "Chemical change or backlash" , "counter-revolution" , "Result of stimulus" .)