The picture conceals a long poem (4)
I believe the answer is:
epic
'a long poem' is the definition.
(I've seen this in another clue)
'the picture conceals' is the wordplay.
'conceals' says the answer is hidden in the clue.
'EPIC' is hidden in the letters of 'the picture'.
(Other definitions for epic that I've seen before include "Impressive scope" , "Nonsense, rubbish" , "Blown up" , "Large-scale" , "Long narrative poem telling of a hero's deeds" .)