Step round city's grand building (6)
I believe the answer is:
palace
'grand building' is the definition.
(I've seen this before)
'step round city's' is the wordplay.
'step' becomes 'pace' ('pace' can be a synonym of 'step').
'round' is an insertion indicator.
'city' becomes 'la' (Los Angeles).
'pace' enclosing 'la' is 'PALACE'.
(Other definitions for palace that I've seen before include "Royal building" , "Royal dwelling" , "Head of state's residence" , "Regal mansion" , "Splendid house" .)