A page gone over for homework (4)
I believe the answer is:
prep
'for homework' is the definition.
(synonyms)
'a page gone over' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'page' becomes 'p'.
'gone over' says the letters should be written in reverse.
'per'+'p'='perp'
'perp' reversed gives 'PREP'.
(Other definitions for prep that I've seen before include "Homework (abbr.)" , "warm-up race" , "School work done outside lessons" , "Homework (abbrev.)" .)