Stress English politician possesses — one collapses finally (8)
I believe the answer is:
emphasis
'stress' is the definition.
('emphasis' can be a synonym of 'stress')
'english politician possesses one collapses finally' is the wordplay.
'english' becomes 'e' (abbreviation).
'politician' becomes 'mp'.
'possesses' becomes 'has' ('have' can be a synonym of 'possess').
'one' becomes 'i' (Roman numeral).
'finally' says to take the final letters.
The final letter of 'collapses' is 's'.
'e'+'mp'+'has'+'i'+'s'='EMPHASIS'
(Other definitions for emphasis that I've seen before include "Stress laid on, eg, words" , "Stress, accent" , "Special importance or prominence given to something" , "Accentutation" , "Importance; priority" .)