In sleep I composed great story (4)
I believe the answer is:
epic
'story' is the definition.
(I have seen 'Heroic story' mean 'epic' so perhaps 'story' could also mean 'epic')
'in sleep i composed great' is the wordplay.
'in' says the answer is hidden in the clue.
'EPIC' can be found hidden inside 'sleep i composed great'.
(Other definitions for epic that I've seen before include "Long adventure story" , "Heroic, huge" , "of impressive proportions?" , "Long narrative poem telling of a hero's deeds" , "Heroic; saga" .)