Slowed down song during act (7)
I believe the answer is:
delayed
'slowed down' is the definition.
(delaying is a kind of slowing down)
'song during act' is the wordplay.
'song' becomes 'lay' (lay is a kind of song**).
'during' indicates putting letters inside.
'act' becomes 'deed' (deed is a kind of act).
'lay' placed into 'deed' is 'DELAYED'.
(Other definitions for delayed that I've seen before include "Caused to be held up" , "Put back" , "Deferred, postponed" , "Protracted" , "Detained, help up" .)