Key restaurant lined with gold (5)
I believe the answer is:
ivory
'key' is the definition.
(I've seen this before)
'restaurant lined with gold' is the wordplay.
'restaurant' becomes 'ivy' (I am not sure about this - if you are sure you should believe this answer much more).
'lined with' means one lot of letters goes inside another (as in being filled or lined with something).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'ivy' going around 'or' is 'IVORY'.
Can you help me to learn more?
(Other definitions for ivory that I've seen before include "Yellowish white - material from tusks" , "Creamy white" , "What may be carved" , "Shade of white" , "Creamy colour" .)