Child's toy muffling bird song (3,2,2)
I believe the answer is:
let it go
'song' is the definition.
'child's toy muffling bird' is the wordplay.
'child's toy' becomes 'lego'.
'muffling' is an insertion indicator.
'bird' becomes 'tit' (tit is a kind of bird).
'lego' enclosing 'tit' is 'LET IT GO'.
(Other definitions for let it go that I've seen before include "Disney tune" , "Hit song from Disney's 2013 animated feature film Frozen" , "Excuse something" , "Refrain from comment" .)