Friend joins expert in grand house (6)
I believe the answer is:
palace
'grand house' is the definition.
'palace' can be an answer for 'house' (palace is a kind of house). I'm not sure about the 'grand' bit.
'friend joins expert' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'joins' says to put letters next to each other.
'expert' becomes 'ace' (ace can informally mean very good).
'pal'+'ace'='PALACE'
'in' is the link.
Can you help me to learn more?
(Other definitions for palace that I've seen before include "Regal mansion" , "Home fit for a king" , "Sovereign's residence" , "Royal building" , "Large building" .)