One's performance needs backing of leading couple for capital (4)
I believe the answer is:
oslo
'for capital' is the definition.
(capital of Norway)
'one's performance needs backing of leading couple' is the wordplay.
'leading couple' can mean two substitutions for 'leading'.
'one's performance' becomes 'lso' (I can't explain this - if you can you should believe this answer much more).
'needs backing' is a reversal indicator (I've seen 'needing backing' mean this).
'leading' suggests taking the first letters.
'leading' says to put letters next to each other.
The first letter of 'of' is 'o'.
'lso' in reverse letter order is 'osl'.
'osl'+'o'='OSLO'
Can you help me to learn more?
(Other definitions for oslo that I've seen before include "Norwegian capital" , "Northern capital of Czechoslovakia?" , "Northern city" , "Nordic city" , "European capital city (found in Czechoslovakia!)" .)