Friend and expert in building (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')
'friend and expert' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'and' means one lot of letters go next to another.
'expert' becomes 'ace' (ace can informally mean very good).
'pal'+'ace'='PALACE'
'in' acts as a link.
(Other definitions for palace that I've seen before include "football team" , "Royal building" , "Head of state's residence" , "Sovereign's residence" , "Castle" .)