Poem sung in act held up (7)
I believe the answer is:
delayed
'held up' is the definition.
('delay' can be a synonym of 'hold up')
'poem sung in act' is the wordplay.
'poem sung' becomes 'lay'.
'in' means one lot of letters goes inside another.
'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 "Protracted" , "Detained, help up" , "Put back" , "Shelved" , "Held up, back" .)