Express's quiet about Conservative power grab (6)
I believe the answer is:
putsch
'power grab' is the definition.
'express's quiet about conservative' is the wordplay.
'express' becomes 'put'.
'quiet' becomes 'sh'.
'about' indicates putting letters inside.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'sh' going around 'c' is 'sch'.
'put'+'sch'='PUTSCH'
(Other definitions for putsch that I've seen before include "A sudden political rebellion" , "political revolt" , "Sudden illegal acquisition of power" , "Violent attempt to overthrow a government" , "Coup dtat" .)