Friend and expert in building (6)
I believe the answer is:
palace
'building' is the definition.
(I know that palace is a type of building)
'friend and expert' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'and' is a charade indicator (letters next to each other).
'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" .)