Cheers holding in extravagant rolling rear (8)
I believe the answer is:
buttocks
'rear' is the definition.
('buttocks' can be a synonym of 'rear')
'cheers holding in extravagant rolling' is the wordplay.
'cheers' becomes 'bucks' (to buck up can mean to cheer someone up).
'holding in' indicates putting letters inside.
'extravagant' becomes 'ott' (I've seen this in other clues).
'rolling' says the letters should be written in reverse.
'ott' reversed gives 'tto'.
'bucks' enclosing 'tto' is 'BUTTOCKS'.
(Other definitions for buttocks that I've seen before include "Get behind" , "Bottom" , "Sit on them" , "Body parts covered at least by 13" , "Rear" .)