Fascinating all-nighter, breaking up around noon (11)
I believe the answer is:
enthralling
'fascinating' is the definition.
('enthralling' can be a synonym of 'fascinating')
'all-nighter breaking up around noon' is the wordplay.
'breaking up' is an anagram indicator.
'around' indicates putting letters inside.
'noon' becomes 'n'.
'all'+'nighter'='allnighter'
'allnighter' with letters rearranged gives 'ethralling'.
'ethralling' placed around 'n' is 'ENTHRALLING'.
(Other definitions for enthralling that I've seen before include "Deeply fascinating" , "Fascinating; enslaving" , "Keeping spellbound" , "Patriotic work by Sibelius" .)