Building a friend one (6)
I believe the answer is:
palace
'building' is the definition.
(I have seen 'Large building' mean 'palace' so perhaps 'building' could also mean 'palace')
'a friend one' is the wordplay.
'a friend' becomes 'pal' (pal can mean a friend).
'one' becomes 'ace' (ace has a value of one in some card games).
'pal'+'ace'='PALACE'
(Other definitions for palace that I've seen before include "eg Buckingham" , "Splendid house" , "Royal dwelling" , "Royal mansion" , "imposing building" .)