City play up in knockout (5)
I believe the answer is:
kyoto
'city' is the definition.
(Kyoto is an example)
'play up in knockout' is the wordplay.
'play' becomes 'toy' ('toy' can be a synonym of 'play').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in' is an insertion indicator.
'knockout' becomes 'ko' ('KO' can be a synonym of 'knockout').
'toy' reversed gives 'yot'.
'yot' inserted inside 'ko' is 'KYOTO'.
(Other definitions for kyoto that I've seen before include "City of agreement on global warming" , "Japanese capital before 1868" , "Old Japanese capital" , "Tokyo (anag) - former capital of Japan" , "nearby city" .)