Friend and expert on 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' says to put letters next to each other.
'expert' becomes 'ace' (ace can informally mean very good).
'pal'+'ace'='PALACE'
'on' acts as a link.
(Other definitions for palace that I've seen before include "Home fit for a king" , "Large building" , "See 23" , "Royal building" , "Stately mansion" .)