One mistake at the end of game somewhere in London (7)
I believe the answer is:
ruislip
'somewhere in london' is the definition.
(area of London)
'one mistake at the end of game' is the wordplay.
'one' becomes 'i' (Roman numeral).
'mistake' becomes 'slip' ('slip' can be a synonym of 'mistake').
'at the end of' means one lot of letters go next to another.
'game' becomes 'ru' (rugby union).
'i'+'slip'='islip'
'islip' put after 'ru' is 'RUISLIP'.
(Other definitions for ruislip that I've seen before include "place in Metroland" , "Town west of Harrow" , "6D 5A [TUBE STATION]" .)