Old London players returned to capital (4)
I believe the answer is:
oslo
'to capital' is the definition.
(capital of Norway)
'old london players returned' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'london players' becomes 'lso' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'returned' says the letters should be written backwards.
'o'+'lso'='olso'
'olso' reversed gives 'OSLO'.
Can you help me to learn more?
(Other definitions for oslo that I've seen before include "Scandinavian port" , "Northern city" , "Capital city, called Kristiania until 1924" , "Go solo to European capital" , "foreign port" .)