Naval chief revised chart to include variable London landmark (9,4)
I believe the answer is:
admiralty arch
'london landmark' is the definition.
(structure in central London)
'naval chief revised chart to include variable' is the wordplay.
'naval chief' becomes 'admiral' (senior naval officer).
'revised' is an anagram indicator.
'to include' means one lot of letters goes inside another.
'variable' becomes 'y' (algebra).
'chart' with letters rearranged gives 'tarch'.
'tarch' placed around 'y' is 'tyarch'.
'admiral'+'tyarch'='ADMIRALTY ARCH'
(Other definitions for admiralty arch that I've seen before include "Feature over The Mall" , "London landmark between The Mall and Trafalgar Square" , "building in London" , "Building between The Mall and Trafalgar Square" , "It stands at the Trafalgar Sq. end of the Mall" .)