Dreamer writhing around in rest (9)
I believe the answer is:
remainder
'rest' is the definition.
('remainder' can be a synonym of 'rest')
'dreamer writhing around in' is the wordplay.
'writhing' indicates an anagram.
'around' indicates putting letters inside.
'dreamer' is an anagram of 'remader'.
'remader' enclosing 'in' is 'REMAINDER'.
(Other definitions for remainder that I've seen before include "Unsold copy of book" , "Scrap" , "Rest - sell off (books) cheap" , "that's left" , "Balance" .)