View through Downing Street? (5)
I believe the answer is:
vista
'view' is the definition.
(synonyms)
'through downing street?' is the wordplay.
'through' becomes 'via' (via means going through).
'downing' means one lot of letters goes inside another (to down food or drink is to swallow it quickly).
'street?' becomes 'st' (abbreviation).
'via' enclosing 'st' is 'VISTA'.
(Other definitions for vista that I've seen before include "Panorama" , "Distant view" , "Pleasant view" , "One gives a long look" , "View (with avenue)" .)