Friend, youngster, meeting one old architect (8)
I believe the answer is:
palladio
'architect' is the definition.
(I know that Palladio is a architect)
'friend youngster meeting one old' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'youngster' becomes 'lad' (a young boy).
'meeting' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pal'+'lad'+'i'+'o'='PALLADIO'
(Other definitions for palladio that I've seen before include "Andrea ---, classic-revival architect" , "16th-century neoclassical Italian architect" .)