Old Japanese capital Tokyo replaced (5)
I believe the answer is:
kyoto
'old japanese capital' is the definition.
(I've seen this before)
'tokyo replaced' is the wordplay.
'replaced' is an anagram indicator.
'tokyo' anagrammed gives 'KYOTO'.
(Other definitions for kyoto that I've seen before include "nearby city" , "Japanese city of treaty on climate change" , "Tokyo (anag) - former capital of Japan" , "Old Japanese capital" , "Former Japanese capital city - Tokyo (anag)" .)