Finally gains pyrrhic victory with the reaper's tool (6)
I believe the answer is:
scythe
'reaper's tool' is the definition.
(I've seen this in another clue)
'finally gains pyrrhic victory with the' is the wordplay.
'finally' indicates one should take the final letters.
'with' means one lot of letters go next to another.
The last letters of 'gains pyrrhic victory' are 'scy'.
'scy'+'the'='SCYTHE'
(Other definitions for scythe that I've seen before include "Tool for cutting grass with blade on long pole" , "Old mowing implement for hand use" , "It's useful for hacking" , "Long-handled cutter" , "Corn-cutting tool" .)