Just one militant will seize power (9)
I believe the answer is:
impartial
'just' is the definition.
(associated in meaning)
'one militant will seize power' is the wordplay.
'one' becomes 'i' (Roman numeral).
'militant' becomes 'martial' (I can't justify this - if you can you should give a lot more credence to this answer).
'will seize' means one lot of letters goes inside another.
'power' becomes 'p' (symbol in physics).
'martial' placed around 'p' is 'mpartial'.
'i'+'mpartial'='IMPARTIAL'
Can you help me to learn more?
(Other definitions for impartial that I've seen before include "not taking sides" , "Showing lack of favouritism" , "Disinterested" , "Fair, unprejudiced" , "Objective" .)