Held up legal document that contains poem (7)
I believe the answer is:
delayed
'held up' is the definition.
('delay' can be a synonym of 'hold up')
'legal document that contains poem' is the wordplay.
'legal document' becomes 'deed' (deed is a kind of legal document).
'that contains' means one lot of letters goes inside another.
'poem' becomes 'lay' (lay is a kind of poem).
'deed' placed around 'lay' is 'DELAYED'.
(Other definitions for delayed that I've seen before include "Held up, back" , "Protracted" , "Made late" , "Caused to be held up" , "Deferred, postponed" .)