Vogue magazine's ultimate decline (4)
I believe the answer is:
fade
'decline' is the definition.
(I know that fade is a type of degenerate)
'vogue magazine's ultimate' is the wordplay.
'vogue' becomes 'fad' (fad is a kind of vogue).
'ultimate' says to take the final letters.
The last letter of 'magazine' is 'e'.
'fad'+'e'='FADE'
(Other definitions for fade that I've seen before include "Grow dim" , "Grow fainter" , "Yellow" , "flag" , "Discolour" .)