John —, Regent Street architect (4)
I believe the answer is:
nash
(Other definitions for nash that I've seen before include "English architect, d. 1835 - American poet, d. 1971" , "John -, Eng. architect" , "John --, British architect" , "Ogden --, US humorous poet" , "Architect of Buckingham Palace, d. 1835" .)