Demote English supporter overcome by speed (8)
I believe the answer is:
relegate
'demote' is the definition.
('relegate' can be a synonym of 'demote')
'english supporter overcome by speed' is the wordplay.
'english' becomes 'e' (abbreviation).
'supporter' becomes 'leg'.
'overcome by' indicates putting letters inside.
'speed' becomes 'rate' (rate is a kind of speed**).
'e'+'leg'='eleg'
'eleg' placed within 'rate' is 'RELEGATE'.
(Other definitions for relegate that I've seen before include "Send down" , "Transfer to a lower division" , "Put in a lower division" , "Move down to a lower grade" , "Consign to less important place - maybe a soccer team" .)