Old musical which contains old piano (3,3)
I believe the answer is:
top hat
'old musical' is the definition.
'top hat' can be an answer for 'musical' (I've seen this before). I'm unsure of the 'old' bit.
'which contains old piano' is the wordplay.
'which' becomes 'that' ('that' and 'which' are often synonymous).
'contains' indicates putting letters inside.
'old' becomes 'o' (abbreviation).
'piano' becomes 'p' (musical abbreviation).
'o'+'p'='op'
'that' enclosing 'op' is 'TOP-HAT'.
Can you help me to learn more?
(Other definitions for top hat that I've seen before include "Formal head covering" , "It is worn with tails" , "Example of elegance" , "Film starring Fred Astaire" , "Kind of headgear, goes with 28 down" .)