One of the three Rs (7)
'one of the three rs' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for reading that I've seen before include "red-brick uni" , "Perusing; university in Berkshire" , "Town west of London" , "Knowledge of literature" , "University - studying" .)
(Other definitions for writing that I've seen before include "Literary composition" , "Penning" , "Producing (a book)" , "Penmanship" , "Putting words down" .)