Antique piano inside — a model (7)
I believe the answer is:
replica
'model' is the definition.
(I've seen this before)
'antique piano inside a' is the wordplay.
'antique' becomes 'relic' (I've seen this before).
'piano' becomes 'p'.
'inside' indicates putting letters inside.
'relic' going around 'p' is 'replic'.
'replic'+'a'='REPLICA'
(Other definitions for replica that I've seen before include "Perfect copy" , "Copy original" , "Parcel I found with exact copy" , "Parcel I used to contain exact copy" , "knock-off" .)