King Edward provides backup in sordid extremes (4)
I believe the answer is:
spud
'king edward' is the definition.
(I've seen this before)
'backup in sordid extremes' is the wordplay.
'backup' becomes 'pu' (I am not sure about this - if you are sure you should believe this answer much more).
'in' indicates putting letters inside.
'extremes' means to remove the middle letters.
'sordid' with its centre removed is 'sd'.
'pu' inserted into 'sd' is 'SPUD'.
'provides' is the link.
Can you help me to learn more?
(Other definitions for spud that I've seen before include "Starchy bit of food" , "Weed spade" , "The common potato" , "Commoner's King Edward?" , "Potato (colloq.)" .)