Playful piece of music lacking intro (4)
I believe the answer is:
arch
'playful' is the definition.
(I've seen this before)
'piece of music lacking intro' is the wordplay.
'piece of music' becomes 'march' (eg a wedding march).
'lacking intro' means to remove the first letter (the word's introduction is removed).
'march' with its initial letter taken off is 'ARCH'.
(Other definitions for arch that I've seen before include "Teasing; part of foot" , "Curved form" , "one that may have fallen" , "This enemy is major" , "Curved bony structure in the foot" .)