The end of war, it's said, is such a small thing (5)
I believe the answer is:
piece
This clue could be a double definition.
'the end of war' is the first definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'it's said is such a small thing' is the second definition.
(a small piece of something)
Can you help me to learn more?
(Other definitions for piece that I've seen before include "Morsel" , "Slice" , "Portion, bit" , "Bit - part" , "man on board" .)