Aggressive speech managed to stop Major's replacement (8)
I believe the answer is:
harangue
'aggressive speech' is the definition.
(I've seen this before)
'managed to stop major's replacement' is the wordplay.
'managed' becomes 'ran' (I've seen this before).
'to stop' is an insertion indicator (stopping can mean blocking a gap).
'major's replacement' becomes 'hague' (I can't justify this - if you can you should believe this answer much more).
'ran' placed inside 'hague' is 'HARANGUE'.
Can you help me to learn more?
(Other definitions for harangue that I've seen before include "Rant" , "urgent words" , "Deliver a forceful speech" , "Forceful and aggressive speech" , "Hectoring lecture" .)