Prepare for an exam (6)
(Other definitions for revise that I've seen before include "Study, learn" , "Go over books and notes again to refresh memory" , "Go over studies again before exam" , "Make changes to" , "Swot for an exam" .)
'prepare for an exam' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for study that I've seen before include "Scrutinise" , "Office at home" , "Small office" , "Something worth observing" , "Writing-room" .)