Contrary to opinion, week appears, ultimately, a long time (5)
I believe the answer is:
yonks
'a long time' is the definition.
(yonks can informally mean many years)
'contrary to opinion week appears ultimately' is the wordplay.
'ultimately' says to take the final letters.
The last letters of 'contrary to opinion week appears' are 'YONKS'.
(Other definitions for yonks that I've seen before include "Slangily, a long time" , "Donkey's years!" , "An age" , "Many years (slang)" , "eternity it seems!" .)