City plaza lies oddly across major road (6,4)
I believe the answer is:
square mile
'city' is the definition.
(both can mean the City of London)
'plaza lies oddly across major road' is the wordplay.
'plaza' becomes 'square' (synonyms).
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'across' indicates putting letters inside.
'major road' becomes 'mi' (resembles Ms, English motorway).
The alternate letters of 'lies' are 'le'.
'square'+'le'='squarele'
'squarele' enclosing 'mi' is 'SQUARE MILE'.
(Other definitions for square mile that I've seen before include "financial district" , "The - - denotes London's financial area" , "City of London (colloq.)" .)