Critical conservative capital about to lead uncontrollable movement (9)
I believe the answer is:
climactic
'critical' is the definition.
(both can mean at the most important stage)
'conservative capital about to lead uncontrollable movement' is the wordplay.
'conservative' becomes 'C' (abbreviation commonly used in UK politics).
'capital' becomes 'lima' (capital of Peru).
'about to' becomes 'c' (I can't justify this - if you can you should believe this answer much more).
'lead' means one lot of letters go next to another.
'uncontrollable movement' becomes 'tic' (involuntary twitch).
'c'+'lima'+'c'+'tic'='CLIMACTIC'
Can you help me to learn more?
(Other definitions for climactic that I've seen before include "Coming to an exciting finish" , "Decisive" , "Coming to a high point" , "Rising to a peak" , "approaching a high" .)