New door with new piece (5)
I believe the answer is:
rondo
'piece' is the definition.
(type of musical composition)
'new door with new' is the wordplay.
'new' indicates anagramming the letters.
'with' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'door' is an anagram of 'rodo'.
'rodo' enclosing 'n' is 'RONDO'.
(Other definitions for rondo that I've seen before include "Musical piece with returning theme" , "Last movement of a sonata or concerto" , "Piece of music with recurring theme" , "Piece in the form of a sonata's last movement, say" , "Piece of music, often of the last movement" .)