Power grab by half-cut schmoe that plonk started off (6)
I believe the answer is:
putsch
'power grab by half-cut schmoe that plonk started off' is the definition.
'putsch' can be an answer for 'power grab'. I am not certain of the remainder of the definition.
'power grab by half-cut schmoe' is the wordplay.
'power grab by' becomes 'put' (I can't justify this - if you can you should believe this answer much more).
'half cut' means to take half the letters.
'schmoe' cut in half is 'sch'.
'put'+'sch'='PUTSCH'
Can you help me to learn more?
(Other definitions for putsch that I've seen before include "Revolutionary uprising" , "political revolt" , "Sudden illegal acquisition of power" , "A sudden political uprising" , "Change of government by force" .)